Yes, they targeted other sites before, however
1) Itch.io's handing of "Oh, we shadowbanned everything NSFW without a moment's notice, sorry (lulz)" is just terrible and 2) Steam is bigger - but has far more "mainstream" games. Steam can easily shrug off losing the handful of NSFW games in favor of keeping the mainstream ones - Itch.io has many more smaller developers and many more NSFW games, so the stakes are different for them: NSFW content likely is a much bigger share than e.g. on Steam, so they should try to fight for that bigger piece, or at least not act in a way that will NOT burn all goodwill towards them (instead of how they did act)
Yea, they definitely should be more professional in terms of the first point and that was always a thing before albeit we don't know how it looked on their end. Maybe they got an actual legal letter and they had to act fast? Some people already have been thinking if they didn't have something serious as a bonus that made them act faster: https://bsky.app/profile/torrent-empress.bsky.social/post/3luowy64rcl2y
Worth remembering that itch.io staff is small. They didn't have ways to check if games are breaking the rules or not and it was put on the users to potentially report those. Now that itch.io has been forced to get rid of certain NSFW games and they can't check it easily, the only thing they could do to comply and not lose a major way of paying developers on the site was to nuke everything and then try to potentially hire people to actually check that stuff or try to somehow do it on their own.
Responding to second point, I don't think they wanted to lose that pie either. For them it's way more of a big deal than Steam because they have been hosting NSFW games for so long but it's really difficult do to much if your own payment processors are literally against what you host and give you an ultimatum, which they shouldn't be able to do in the first place. That is the true problem that needs to be resolved, because otherwise it will keep happening and they will start leverage their power to censor something more than just controversial porn.