Pretty much what FrostShot said. Ever since Season I, I've really only seen open season declared on LoL by the other MOBA communities. There's also YouTube videos of why DotA is the best and League is made for little kids. The most reaction I've seen in the opposite direction would be something like "DotA? Get that shit outta here!" which is pretty tame. You'll find similar reactions between the different playerbases from comments on MOBA reviews. League can have some really toxic players, but they do get banned, suspended and warned. Riot actively and passively encourages positive play and player experience and actively and passively discourages the opposite. There was a video I saw on YouTube about relating Riot's techniques to the real world and how it might positively affect society (which it acknowledged was just conjecture, but it was interesting).
As for why play instead, League has a lot to offer. I've played DotA Allstars, Heroes of Newest and DotA 2. HoN was just a straight-up copy of Allstars with some ability shuffling. DotA 2 is a legitimate copy of Allstars. All of them rely on Strength/Agility/Intelligence stats as their core gameplay. League doesn't use that model, and in my experience its the only one that does what it does with breaking down all the different stats. So there's some unique experience. There's a plethora of fanart and fanvideos. Seriously, it has a pretty ridiculous amount of stuff made by fans that other fans can enjoy.
Another note on communities: Allstars had one of the worst communities. Games were hosted in conjunction with bots. These bots had a list of accepted players. That is, not new players. If you were new and you joined the game, you were kicked, no questions asked. The only way to be able to play was to find a game not host-assisted by a bot or get someone who played to vouch for you. However, if you ever did something the host didn't like, like have a poor game, they could shitlist you, banning you permanently from playing any Allstars games in the future. Then you were left with registering another WarCraft III account or trying to jump through stupid amounts of hoops to get removed from the shitlist.
DotA 2 inherited that community.