Knowledge Gap

Dota 2 and Dota All-Star had more or less the same level of complexity, but lots of people did not really know how to play Dota All-Star. Just being a one-trick pony like a split-pushing as Nature's Prophet or knowing a basic farming triangle as Anti-Mage was enough to carry the game.

Dota 2 however came to life at a time streaming was a thing. Knowledge was transferred at breakneck speed and what was once advanced knowledge now is treated like basic information everybody should know. 

My point is... you had to learn a lot less to stop being a noob in Dota All-Star, and if you played Warcraft III you already had a very basic idea of what some stuff did. Dota 2, while being similar, expected more of you.

And now comes patch 7.33 with a bigger map, new buildings, and of course new items and heroes. If you play constantly this knowledge is slowly absorbed but if you take a break or god forbid, are brand new to the game, that's a lot to learn before you can kill an Ancient.