Perhaps this is a way to get feedback from players without having to worry about a ton of negative reviews to have to overcome should the game not be initially well received. Undergrad Steve had that problem on his new game, though I feel like that game deserved them.
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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 :)
I suspect Undergrad Steve felt like the success of College Kings would give him a huge reservoir of good will for the release of Unscripted, but that wasn't the case. Admittedly, the situations are different because he handed the writing of CK off to another team while he worked on Unscripted and the results were so bad that he is going back and redoing a lot of the CK content that they wrote. My point is that it takes a long time to build up trust with customers and a very short time to lose it. Plus, given the lengths of time between releases for episodic AVNs, the mistakes have a long time to sit in peoples' minds.