Broadcast television still concentrates pilot production into a narrow winter window. The compression exists because the money that funds the season is committed on a fixed calendar.
Advertising is sold before the shows exist
Networks present their upcoming schedules to advertisers in the spring and sell a large share of the year's inventory at that point.
To present a schedule, a network must know which shows it is ordering, which means pilots have to be shot and evaluated beforehand.
The advertising deadline therefore sets the production deadline, and everything upstream is arranged to meet it.
Everything competes for the same crews
Dozens of pilots shoot in the same weeks, drawing on a limited pool of crews, stages, equipment and available actors in a few production centers.
That concentration drives costs up and forces compromises, since the best available option in a crowded season is not the best option generally.
Productions that shoot outside the window enjoy easier access to resources, which is one argument made for abandoning the system.
Actors are held under short agreements
Performers cast in pilots sign agreements that commit them to the series if it is ordered, keeping them unavailable to other projects during the decision period.
Those holds expire, which creates further pressure to decide quickly, since an expired hold can lose a network the cast it built the pilot around.
The clock on availability is often what forces a decision more than the evaluation of the pilot itself.
A pilot is a poor test of a series
Pilots carry the burden of introducing everything at once, so they rarely resemble the show as it will look in its tenth episode.
Networks know this and read a pilot partly as evidence about the people who made it rather than as a representative sample.
Series that improve markedly after their opening episode are common enough that the limitation is widely acknowledged inside the process.
Streaming avoided the calendar entirely
Services without a fixed advertising presentation order series directly from scripts and produce them whenever resources allow.
That freedom removes the seasonal bottleneck but also removes the pilot as a checkpoint, committing far more money before anything is seen.
Broadcast networks have adopted some of this flexibility, ordering shows outside the window, while keeping the spring presentation that created the system.