Itch.io is about 5% of the size of Steam, so obviously getting tons of negative reviews there hurt more.
One aspect of this is that itch.io has good Patreon integration; I offer Steam keys to Patreons for testing, but a lot of people don't want adult games in their steam library (in case friends or family can spot them being a perv). They usually don't have the same issues with itch.io, so giving them an itch.io key is an option.
Another aspect is that Steam treasures wishlists a lot; it uses them to determine if you're a "real game" at launch, and if you have sufficient (~5-10K) at launch, they give you a ton of visibility. Love of Magic Book 1 had 6K wishlists at launch, which translated into 2 million 'views' (times Steam showed the game to players that might be interested). As such, waiting until you're ready on Steam makes a lot of sense.
The last aspect is... I always do itch.io releases first :) I started doing adult games as a hobby, and putting up an itch.io page was how I shared my game with the first players. That worked well, and I like to repeat things that work well :)