Basically, because Wizards of the Coast came up with the core concepts of the pay-to-win money treadmill game 15 years before the App Store launched--low cost to start playing, psychological impetus to spend more money on a regular basis, appealing to kids and teens with lots of free time and a willingness to badger their parents for money, social enough that players will stay in the game for years because all their friends play. It's kind of breathtaking when you look at the parallels now.

They had that entire previously-untapped goldmine of a market space to themselves (or near enough, given the quality of their early competition) for long enough to get really culturally entrenched, and that meant they were able to shift the model online (and keep a much greater chunk of the profits) without as much resistance as you might expect--if you already believe that 5 cents' worth of ink and cardboard can be worth dozens or hundreds of dollars if it's got the right picture on it, it's a lot easier to believe that the same picture on your computer screen can be worth the same.