It one main reason - but NOT only reason. I dont think you can reduce people to single-issue voting simpletons or robots. Humans consider severals factors before settling down on best way forward.
Kibaki in 2007 was PORK and gema were defending their gov. In 2013 - Uhuru become ICC hero for defending the community during PEV and when he nailed Ruto - he had real shot at power - GEMA backed him. Ruto has been loyal and reliable friend to many in Mt kenya for 10yrs - speaks the right language (hustler thing) and name it.
Otherwise Kalonzo supported Kibaki in 2007 and had no time for Raila until last minute - but nobody felt him in GEMA.
So anyway for now I would say 1/3 Ruto support is protest vote against Uhuru, 1/3 is core hatred of Raila, and 1/3 is merited (Ruto has spend time, messaging, tactics, campaigns and money).
If Uhuru had backed someone else - like say Kalonzo or maDVD - I dont see how Mt kenya would have abandoned Ruto enmase.
But definitely they would be going even 50-50 with Ruto. Raila is hopeless because he start from -VE position. Others start from zero.
What Uhuru has done is to get Raila to zero
otherwise he was negative. Martha may succeed in getting Raila hapo 10 percent
but given Nyeri today
- hiyo nayo inaweza kuwa ngori
GEMAs decision has been to vote against Raila . Alost 17 years now.
Its not that they liked Kibaki, in 2007 Its not that they liked Uhuru in 2013 and 2017 . Its not that they like Ruto. Its simply they vote against Raila.