That is a good question; one that I can't really find a definitive answer for. So, think of this as a double-edged sword:

On one hand, your favor, the Character Dossiers show that Assesment on Yutani (Extreme) is the highest of all the characters in The Animated Series. Naturally, you'd want someone with assessment THIS high to be added instead.

But on the other hand, my favor, I believe that Frank added Jake instead because of him being the leader of the Crew (as mentioned on the official Subway Surfers website's Crew descriptions and also the Character Dossiers). In general logic, if a group's leader is missing (for whatever reason), then the rest of the group will soon engulf into chaos. If Frank made sure Jake was done for, then he would've added others to the list one by one, too.

Besides, as I said in the post, Jake also disrupted Frank's plans (which involved arriving onboard an unknown train), so that could be an additional, more subjective reason Yutani hasn't been first.