Hmmm, maybe it is because we are the largest MOBA community in the history of MOBAs beating the second place MOBA by over 60 million players?
When you get such a diverse group of people, especially behind an anonymous keyboard, one is bound to find these toxic individuals. Even in real life, large communities such as this rarely escape conflict amount another.
So yeah, maybe HotS might be toxic right now. My friend played it in beta when only about 10,000 people played it, stating how nice everyone was. If that game were to ever reach the popularity and community size of LoL, then the Team XP share might cause it to be one of the most toxic games out there, since if one person on the team is bad, or DCs for a little, then the whole team suffers. AFKs and DCs have a larger impact in that game.
Also, DotA is considered to be a more competitive game with a higher learning curve than LoL. In result, one can often be pressured to be coached and can be raged at quite easily if a mistake is being made. Being a casual player is quite difficult to be there, unless playing with friends. It is practically required to be serious, or to not play at all. This sort of attitude tends to provoke more negative behavior.
My little brother plays CoD a shit ton, which has a larger fanbase than LoL, and they have coms. The amount of toxicity and slurs my little brother hears every single game was more than anything I heard or saw in LoL. Experiencing this is what made me support Riot's view on not supporting in game voice coms.
Just my two cents.