Perhaps it is a psychological result of the situation in the last two decates, where the conservatives seemsee themself as the weaker side, albeit still capable of scoring wins sometimes, and so they understand the importance of consolidation. Admitting that you are a weaker side also let (American) Right to adopt "Don't punch Right" approach. The rationale being, you are not 100% in agreement with another conservative who happen to be trending currently, but they are better than any liberal, and if you show in-fighting then liberals win, since they do that by default.
On the other hand, liberals also see themselves as a stronger side, so they aim to get the best piece of the pie which they see as already won, so the only question is who gets to eat it among fellow liberals. this may go such great lengths that they will occassionally lose to the weaker conservatives because of this. Perhaps this is not even seen as a problem because they feel they may score whenever they like it, so it may be better to yield to a conservative (and spend a term bashing them) than to a fellow, but a different flavor of, liberal.
The apparent difference is strength seems to be due to the fact that the great majority of media (news outlets, cinema, arts and music, video games) is now liberal-leaning, a lot of being virtually a loudspeaker for their programmes.