A gap of two or three years between seasons has become normal for expensive drama. The delay is the sum of several sequential processes, none of which can start before the previous one finishes.
Writing starts after the renewal
Platforms generally decide on renewal after a season has been released and its performance assessed, which takes weeks or months.
Only then are writers hired, and a season of scripts takes many months to develop even with a room working continuously.
Nothing before that point can be treated as certain, so producing scripts in advance is work that may never be paid for.
Series confident of continuing avoid part of this by writing through the gap, which is why a small number of shows return annually while comparable ones do not.
The cast has gone elsewhere
Performers do not wait for a decision, and by the time a series is renewed the principal cast has committed to other projects.
Reassembling them requires finding a window in which everyone is simultaneously available, and each additional principal makes that window harder to locate.
Directors, cinematographers and department heads face the same problem, which is why crews change between seasons of the same series.
Post-production is longer than filming
A series with substantial visual effects requires months of work after the last day of shooting, and that work cannot be compressed by adding people beyond a point.
Effects are distributed across vendors who have their own schedules, and a shot returned unsatisfactory goes back into a queue.
Sound, music and grading follow, and each depends on the picture being locked, so they cannot run in parallel with the work preceding them.
Release timing is a separate decision
A finished season may still be held, because platforms schedule releases to spread their strongest titles across a year.
Awards eligibility periods influence this, as does avoiding a competing release from another service.
A series can therefore be complete for months before anyone is permitted to see it, which is invisible from outside.
What the gap costs
Audiences forget plot detail, and a proportion do not return at all, which means each season begins by rebuilding an audience the previous one had.
Recaps and expositional dialogue expand to compensate, taking time from the new material.
Momentum in the wider conversation is the harder loss, since a series absent for three years is competing for attention as though it were new.