Television

Broadcast/cable TV had a schedule to fill. More than anything else, they needed to fill time slots. They planned around a calendar so annual seasons were only natural. That's also why they tended to have longer seasons, because it's easier to keep viewers coming back on schedule when they are already interested in a show. Plus it's cheaper to to make one long show than two short shows. It leads to brutal production schedules and massive creative compromises, but it fills airtime and that's what counts.

In other words, the economics of old school TV were about filling time while maximizing ratings and minimizing costs, which incentivizes making a smaller number of shows with longer seasons and regular releases.

Streaming has very different incentives. They don't care about filling a schedule, they just want you to subscribe. Every new show is a potential draw, and the more titles they have the more likely it is that one of them will appeal to you. They are incentivized to fill their catalog, not their calendar. As such, there's nowhere near as much pressure to churn out seasons like clockwork. If anything, they probably get more bang for their buck by waiting longer between seasons so that the release of each season is more of an event.

Hell, it doesn't even matter that much if it hurts the show in the long run. As long as they keep getting subscribers and have new shows (or popular old shows) to pull people in, they don't really care if an existing show dies.

mostly because of bigger budgets making scheduling harder. higher profile actors, bigger crews, potential location filming, more time required for effects and post production.

I also heard writing takes longer because it has to be refined and tightened to fit in shorter seasons.

covid and writers strike also made everything worse for a while.

and part of it is probably intentional trying to milk the golden goose for as long as possible. apologies for the mixed metaphor.