Maybe I misunderstood your question to begin with:
Are you maybe asking why they have such a strong focus on the existence of the soul? If so, that is a good question. I think that many of us think of certain topics as being “silver bullets” such that they, themselves, can serve as debate-enders. I know that I’m guilty of pursuing discussions using those aspects. I think that many think that if one convince others of the existence of a soul (a spiritual aspect of a person apart from the physical person), it may be an aspect that cannot be explained from an evolutionary perspective.
It’s probably quite ineffective, because it avoids the original debate and just changes one topic for another. Now instead of questioning whether or not evolution can account for the variety of life we see, an argument over whether or not people are “spiritual beings.”
That’s my guess, anyhow. Maybe not that the presence of a spirit or soul will cause doubt for an evolutionary biologist, for instance. But maybe that it would be cause the layperson, like me, to doubt the efficacy of evolution’s ability to explain all life (including soul and spirit.)