Hearthstone is owned by the behemoth company blizzard and can afford to do stuff like that because its costs are offset by other games especially WoW. Its the same with how Dota 2 does shit like free heroes because its owned by the behemoth Valve which gets a metric fuck ton of money from every game on steam as well as the community marketplace.
Also the more likable beginner experience. League of legends is the largest MOBA on the market. Why? because the beginner experience isn't all that bad. when compared to pretty much every other moba, league's beginner experience is incredible.
and the masteries argument is just.... no. you can get a keystone by level 18. keystones are really the be all end all of masteries.
Also runes..... its a good thing because new players should NOT buy runes until they understand them. when i was a new player i played AP ashe because i though that ability power just meant my abilities did more damage. I didnt understand the game, hell i didnt understand the game until level 12-13. i bought my first rune at level 22, this was because by this point I had enough of a grasp on the game that i could figure out what i was doing.
Personally i think runes should be locked until level 30, but most people disagree with that. saying that new players get these things to slowly is kinda meh, they already did a revamp of the runes and its way cheaper to get a full rune page. Also you have to understand how much moving to a 100 level system would fuck with the matchmaking of low levels.
I remember queing with my level 5 friend (im level 30 at the time. you may ask why and the answer is that about 60% of the sub level 30 players are smurfs so i decided id help him out and power level him to 30 so he could actually play against players around his skill level) and with a level 5 and a level 30 at a silver mmr, the games came out to be pretty even. switching to a 100 level system would throw this on its head.
Also you cant forget the daunting task of reaching level 100 to unlock more of the game. We would know that its like 20 more wins, but to a new player 30 levels feels achievable, but 100 is terrifying. Its why I dont play MMORPGs. it feels like ages to get to the high levels where you actually get to do something other than grind. having 30 levels is better for the feeling of achievability for new players