The 50 Million Dollar Question
It's the $50 million question:  Why is Hearn putting the brakes on?  Let’s play with some scenarios here.  None are factual, but all risk playing out.  It’s likely that nobody aside from those in the inner circles of team AJ know quite what is going on right now, but let’s take a hypothetical look at the outcomes.
Starting with what we do know.  Hearn offered Wilder $12.5 million to come to the UK.  Wilder (backed by DiBella, Haymon and Finkel) offered AJ $50 million to go to the States.  We won’t argue here where the money comes from, is it all real etc, because none of us know.  For the sake of this, we’ll assume it is all in an escrow, ready to collect.
On face value, $50 million for one man to unify the heavyweight division is great news, win or lose (let’s assume there is no draw here).  Everyone gets their cut; Hearn as Promoter, McCracken as Trainer, AJ Boxing (there are a lot of mouths to feed around the topless table) and then finally AJ himself.  For Anthony Joshua, the deal seems a no brainer.  So why (remember, we are assuming the money is in escrow) is Hearn, as Promoter of Joshua, not jumping for joy?
There are key commercial partners that Matchroom need to keep happy.  Start with the big one, Sky TV.  If the rumours are true, for every Pay Per View that is sold, Sky take 50% of the money.  Don’t forget they have the production costs, the outside broadcast from Wembley or Cardiff for a Joshua fight.  These things aren’t cheap and they want their money back.  Also, they hand Matchroom a boxing budget to put shows on, either PPV or Fight Night.  If team Wilder fork out $50 million, then they are also going to be expecting the ability to sell their expensive product to the highest bidder.
Hearn tells us that over 1.5 million PPVs were purchased for Joshua vs Klitschko.  At a going rate of approximately £17 per purchase, taking in 50% of the revenue, that’s £12.75 million that Sky will have raked in (of course there are costs to offset, but they are still making a tidy profit here).  So what if Wilder’s team pay their $50 million and then offer the fight to the highest bidder?
As the broadcast partner to Matchroom and the platform Joshua has built his career on, the risk of losing him at his biggest moment is barely worth considering.  But conversely, Sky also won’t want to be tied into a bidding war.  Every PPV platform in the UK will have an interest in this fight.  Remember, only this month BT Sports launched their own PPV network.  Here is a hypothetical scenario to consider; what if they set their PPV network up to cater for Joshua vs Wilder?  Of course that is unlikely, but not impossible. 
The outcome of paying $50 million for the fight means that Hearn could lose control of who beams the fight into British households.  That outcome is one that might leave massive dents in the relationship with the Sky businessmen.  Outside of a handful of novelty acts (Haye vs Bellew), their ability to reap money through PPV is limited if Joshua isn’t on board.
It is worth noting that Showtime have a solid relationship with Wilder’s team, having broadcast a number of his fights including his most recent with Luis Ortiz.  It was May 2016 that Joshua himself signed an exclusive contract with Showtime ( so it may be that there are bonds that can be called upon.
The next commercial partner to appease is Stubhub.  Much maligned by boxing fans for the secondary ticket market, they are the main primary ticket sellers too for Matchroom.  Take an AJ fight at Wembley or Cardiff.  80,000 people pack the venue, each of them paying a booking fee on top of their ticket price.  Having had a quick look at the pricing for Haye vs Bellew, it’s an educated guess to say the average buyers booking fee is around £8.  Let’s apply that to Cardiff:  £8 x 80,000 = £640,000.  Let’s not forget the resale market.  Say conservatively, 20,000 tickets are resold and the booking fee is then paid again.  £160,000 can be added on, bringing the total to £800,000.  Matchroom won’t be getting the service for free you can assume; typically there is a 15% seller fee, but lets hypothesise that they get mate’s rates.  Say it’s 10% on primary tickets.  That’s taking this total well over £1 million that Stubhub won’t see if Joshua fights away from the big stadiums of the UK. 
Even outside of commercial partners, there is one HUGE problem for Hearn to overcome in letting this fight go to America.  Ego.  How long have we been told that the UK is the home of boxing?  How long have we as fans been spun the narrative that the big money for boxing is on these shores?  Then out of nowhere, the most profitable fight in boxing AND involving a British fighter, ends up in America?  It happens without Hearn at the forefront.  It happens with him looking in.  He becomes a backstory, no longer the attraction piece at the top table.  That’s a tough pill to swallow.
There is one final problem to overcome here, which is possibly the most significant of them all.  Anthony Joshua’s contract with Matchroom apparently expires this year.  Some say August, some say July, some even say November.  The month is unknown, the year seems common knowledge.  There must be tension.  There simply must. 
Hearn would prefer, understandably, the fight to take place in the UK.  That way he controls it all and has the best opportunity to control commercial aspects.  It is a risk (as outlined above) to allow those in the US to dictate terms.  On the other side, Joshua will want to look after number one.  Can Hearn promise him $50 million to fight in the UK?  Why would he accept any less, given that he can be paid that in the US?  It wouldn’t make sense and he is the one doing the work, of course he wants the rewards. 
With the contract between Matchroom and Joshua presumably being discussed, the last thing that Hearn needs right now is tension.  If he were to try and stand in the way of a potential $50 million payday, does it potentially damage the chances of re-signing his marquee name?  THE marquee name.  You can assume yes.  It leaves the significantly sized elephant in the Matchroom HQ sized room; what if Joshua does it without us?  Getting a percentage of something right now is better than getting nothing if Joshua went elsewhere in 2019 and took the money with him.
So what is the outcome?  No idea.  It seems as if Hearn is backed into a corner.  Take the offer and risk upsetting your commercial partners.  Try and put the brakes on the offer and risk upsetting your number one commodity.  Neither seems desirable.  To do the latter, you had best hope there is some way of finding $50 million to pay Anthony Joshua on his terms in the UK.