His politics per se don't have anything to do with it (might be different if he was a complete extremist, like a Conservative ).
But he has hardly had a glitteringly successful political career. Jumping (being pushed?) from one party to another does not demonstrate consistency and commitment, nor does it result in electability. I don't recall him being able to bring the Scottish Unionist Party into the mainstream and make it electable. 225 votes for him the last time he stood (9th out of 10 - 10th place got 224 votes!) hardly demonstrates that he can bring success to the RST.
In a recent speech to the SUP, Mark stated "we have to modernise the language of Unionism so that we are seen as relevant now and in the future and not yesterdays men seeking to turn back the clock".
Indeed Mark, indeed.