Because they are psychologists, it really depends on what is meant by Conservative. If they are Conservative, as in socially conservative and are going to be opposed to homosexuality and transgernders (such as NARM or NARTH or whatever it's called), or automatically assuming porn is bad for the individual or a relationship, or taking a stance that will push their conservative views above all else, they have no business being in the practice of mental health. Religious conservatives can be ok, but only if they do not push their religion or religious practices on others, and tell a patient they must follow religious rituals to get better. Fiscal conservatives can also cause alot more damage than good if they are working with the poor and chronically underemployed and unemployed by telling them they just need to try and work harder.
Really, I would have to say a big part of it is that in psychology there is no room for telling people they must accept Jesus, they must not be gay, or that they must pray. And you can't just assume that someone is poor because they aren't trying hard enough. And of course there is zero room for the delusion that your culture and country are the best ever and everyone else should strive to be like you. I would suspect this is probably the key reason there is such a divide in the social sciences. People can believe them, but they can't push them on others, which is an area a lot of conservatives have troubles with. And because fields like anthropology and sociology require an open mind, I think if we were to really break down this divide we would find the fault has more to do with Conservative ideology than the social sciences themselves.