The old video arcade games "Daytona USA" under the SEGA name had the exclusive rights to everything related to Daytona being in a video game. I actually have the PC version with the cool sound effects for Daytona USA game, but the point is that when other Nascar games that had better graphics came along with Sierra and Papyrus they could not use Daytona track as one the tracks.
I think Daytona was not seen in nascar video game until EA. I tried finding a link on the net that talked about the whole video game licensing problem dealing with "Daytona USA", but I could not.
My point is that I suspect that Geico has a licensing agreement with their Ad company. I recall reading that all the Ad ideas for Geico including the lizard and the caveman come from one company with strong ties to Geico. My guess is that this Ad Agency has licensing control of T-shirts relating to Geico in media and the problem is the ad company not really Geico. I could be an issue of royalties and licensing fees. However, I doubt it.
The Ad company probably thinks drunk rednecks driving beaters with T-Shirts reading "it is so easy a caveman can do it" do not make for a positive image for a car insurance company.