NA kinda killed the brand at the time and if you get offered X amount of money you could invest back into more popular brands then it's just down to business at that time. Maybe they needed the cash injection? Look at MOTU right now, it's not exactly on the scale of Star Wars, Transformers and Ninja Turtles is it? The company that bought the MOTU entertainment rights in 2004 as part of the Filmation library went bust a few years later for overvaluing the brands they were acquiring. So maybe Mattel was wise to sell the rights in the 90's? Fact is you will never know the full story behind it as nobody is going to disclose it.
Talking of NA, wouldn't the entertainment rights for MOTU have been left with whoever made the New Adventures of He-Man cartoon? Perhaps Hallmark gained the MOTU entertainment rights through the purchase of the NA series?