A first season is judged on measures that are mostly invisible to viewers. The premiere audience is one input among several, and rarely the decisive one.
Completion matters more than sampling
Large numbers of people will start almost anything that is promoted heavily. Far fewer finish.
The share of starters who reach the final episode is treated as the honest signal, because it reflects the show rather than the marketing that pushed people toward it.
A modest premiere with a high completion rate is a stronger case for renewal than a large premiere that empties out by the third episode.
Viewing is counted over months, not days
Shows accumulate audience long after release, through recommendation, word of mouth and later promotion.
Decisions therefore wait for a window in which that tail can be measured, which is why renewals are often announced well after a season has finished.
A show that grows steadily across that window is treated differently from one that peaks immediately and decays.
Who watches counts as well as how many
Commissioners look at whether a show brings in viewers who were not already engaged, or holds subscribers who were considering leaving.
A series that mainly entertains people who would have stayed anyway adds less than one that reaches a group the service struggles to reach.
This is why shows with smaller audiences survive when those audiences are hard to acquire by other means.
Costs are compared against the second season, not the first
Renewal arithmetic looks forward. First seasons carry set construction, casting and pilot costs that will not recur.
The relevant question is what another season costs now, against what this audience is worth now, and that figure is usually lower than the first-season spend.
Against that, escalating cast salaries push in the other direction, and a cast that has become expensive can end a show that is otherwise performing well.
Contracts set the deadline
Options on cast and key crew expire. When they do, everyone is free to take other work, and reassembling the show becomes far harder.
That fixes a date by which a decision has to be made, sometimes before all the viewing data is in.
Producers manage the pressure by writing an ending that can serve as either a finale or a hinge, so the show remains viable whichever way the decision falls.