Alex Trebek was synonymous with Jeopardy for a VERY long time.
I would think this test phase was to see what would be best for the show long term.
Ken Jennings was just an average person until he became the king of Jeopardy players.
He has now become synonymous with Jeopardy.
Mayim Bialik has been a well known actress most her life, she is known for that. I think it was her intellect that drew the game show to have her host. But she will never be the face of the show, never be synonymous with the show and will likely continue to work as an actress with Jeopardy being a small side gig.
Also during the strike she said she would not fulfill her duties to Jeopardy so Jennings was brought in to host for her instead. That was likely the final decision maker. Having your host able to work when scheduled is important.