In my opinion, the value for stadium naming rights aren't there on certain stadiums. Why pay a fortune to a club, one that would be criticised by its fan base for doing so, when everybody will just call it by its actual name anyway. Do you call St James Park the Sports Direct Arena?
I have no doubt they have been approached, but Old Trafford, Anfield etc, a huge part of their current income is based around the name of their ground. Old Trafford is worth more to them than any feasible income from being called Chevrolet Park, or Liverpool's Anfield being called the Nivea Face Cream ground.
As at Everton, Goodison Park is part of our identity, it is part of who we are. BMD hasn't yet had the opportunity to be that, which is why, imo, the value potential of naming is so high. If you get in and secure it for 10/20 years at a huge cost, there is a real chance that the name becomes synonymous with the club and your brand name is mentioned socially when your aren't even paying for it any more. That 10/20 year cost is all of a sudden spread out for 30/40 years value. That is something difficult to remove with a simple new name. Similar to the well known story of Hoover and vacuums. Vax/Shark/Dyson don't build Hoovers, Hoover do.