Min check-in of 15-30 mins can be done almost anywhere with limit dependent on terminal size. SEN do not set a limit, the airlines do. Nothing to stop Loganair for example coming along and offering 20 min check-in.
It comes down to the airlines. On the one hand it’s clarity and simplicity, you want a clear message rather than array of times for different airports. On the other hand it’s a cynical bit of cash generation, close earlier than you need and you’re more likely to get extra cash from pax for rebooking. Ryanair is a clear example of a company with standard times whether it’s only flight of the day from Castellon or one of hundreds at Gatwick.