Because from a layman point of view these types of cases have been brought numerous times and they have always been thrown out for lack of standing.
The only cases which proceed are those which argue a different legal issue.
Only other candidates can sue on the citizenship question.
All the cases that concerned the fraud now in the Oval Office were brought after he was elected. That is different from a case brought at the state level under the laws of a state at the point where a candidate has just been placed on the ballot and the complaining party filed under state laws providing for challenges to placing a candidate on the ballot. So far in this cycle there are reports of several such ballot challenge filings. In the Illinois case the opinion of the ballot commission, conclusory as it was, found that there was standing. I believe the same is true with a ballot challenge filed in New York although the ballot commission ducked and said that an actual court would have to decide.