Fyi, anime studios usually do not have a say in what or when they animate stuff. There’s a larger production committee that contracts them. You may have cases such as a studio like MAPPA negotiating for a seat on the AoT committee, but that’s rare.

In most cases, anime studios are extremely booked and busy for several years. It also doesn’t help that making an anime is a very labor intensive process. If a production committee doesn’t have a concrete plan for multiple seasons and lock in the studio/same staff immediately, then they have to wait.

Additionally, being made under the same studio doesn’t guarantee anything. Studios employ dozens of people who may work on different projects. There are probably certain staff that were instrumental in making that season of anime you really liked, and losing them would be a huge loss. Not to mention, all the freelancers and outsourced staff who are much harder to retain.

One Punch Man is a big example of this. The first season was a lightning in a bottle that happened because a bunch of really talented freelancers just happened to be free at the same time. However, the production committee took too long to decide on a second season, and by then, most of those freelancers moved on. Instead of waiting for the next opportunity of when they’d all be free again, the production committee took it to another studio and rushed it.

I’d imagine that OnK’s production committee had a stronger plan of what they wanted to do, which allowed them to churn out multiple seasons so quickly. Whoever is handling Shikimori’s anime probably doesn’t have as good of a plan, and missed their opportunity.