Because getting such warrants on fraudulent info is a no no.
Who says it was fraudulent? Obviously the FISA court judges felt it was pertinent to the case.
They weren't given all the information about it, so he could they?
That has not been verified or even shown to have been necessary. Obviously it's more important that the information be correct than the motives of the informant are. Whether the DNC or Clinton indirectly paid for it is irrelevant if the information is correct.
We know that it obviously was as renewals require that the previous surveillance must be shown to have garnered new information. It was renewed another three times after the application highlighted in the memo.
When judging the credibility of the information, motive most certainly enters the picture. Why do you think partisans argue about who funds political polls during campaigns?
The FBI determined the credibility of the information before including it in the application. Obviously it was credible as the warrant was renewed another three times.
They did not disclose all the information about the source of the application.