I think you're making a mistake in assuming that Texas loved LBJ -- especially Dallas.
Well, if I made a mistake, JFK made a bigger one for not choosing a running mate who could help him win an important state that could go either way.
Could LBJ help win Dallas? Not really. It was conservative and very much against Civil Rights. But Texas on the whole was not so extreme. Texas voted for the Democratic candidate for President throughout the 1960?s, unlike the deep South. LBJ was helpful in swinging Texas JFK?s way in 1960 and would be again in 1964, if he was still on the ticket.