Because it would be politically beneficial for him then. Everyone knew Souter was a liberal, but it didn't stop Bush the Sr from nominating him because it gave him some political leverage. Bush is now sending conservatives but not sticking up for them because he does NOT want to see them sitting on the bench and overturning his liberal policies.
Your analogy does't work because the Senate was in Democrat hands when Bush 41 nominated Souter. Your factual assertions are also incorrect. Although his personal opinions were largely unknown, Souter was a lifelong Republican and known as a strict constructionist. His nomination was bitterly opposed by NOW and a coalition of 38 civil rights organizations, including the NAACP; the opposition was lead by Sen. Edward Kennedy, all the usual liberal suspects. Souter fooled everyone. I know from personal observation that life tenure can have a dramatic effect on judges.
So you are arguing from false premises and a faulty analogy to claim that Bush 41 knowingly nominated a liberal justice and that Bush 43 harbors a secret desire to do the same.