Partly because he didn't want to take part, but also because he wasn't allowed to take part. Participation was dependent on signing the RNC oath of fealty
Thank you for mentioning the RNC oath of fealty, I wasn't familiar with that so I looked it up and it requires the candidate to back whomever the party's eventual nominee is (including if it's Trump). Since he hasn't agreed to back anyone but himself, it makes sense why he wouldn't want to sign it.
It also makes sense why each of the candidates in OP's photo raised their hands in support of Trump if he was the candidate, technically they already pledged the exact same thing.
God it feels good to know that his announcements don't reach me anymore. I literally don't know what he's up to until I incidentally hear in the news that hes being charged with something.
There's zero benefit to Trump in participating in the debates. He's way, way out in front of the other Republicans, it could only serve to hurt him. Instead he did a taped Tucker Carlson interview that I think they aired at the same time (I didn't watch either one).