I think the reason is that Xenforo is a totally managed solution. The Xenforo staff does all the "technical" stuff and DPR staff doesn't need to touch the backend. Hardly the frontend either. They probably don't have access to more than a web interface where they can make minor changes.
I assume the old forum software was patched so many times it it was unwieldy. I don't know what language it's written in, but making old, ancient even, scripts work nicely with updated interpreters on the server is no easy task. I imagine they would have needed to hire one or two programmers up to the job.Easier and likely cheaper to pay Xenforo. Rumor has it the guy that previously handled the forum quit recently. Hence the situation needed an urgent solution.
I think it's as simple as that. They didn't want to deal with the forum so they "outsourced" it.
I don't remember if I already said this here or if it was at dpr. Sorry if I repeat myself. But I think the worst thing is that they destroyed old threads. Longer threads are no longer readable, not even if you really try. Often there is no reference to the post people answered to and the individual subject lines are gone. It was common to change the subject line when a thread forked. I wish they had just archived the old forum. Then it would still be readable and useful. Because there are decades of information there that can still be useful and valuable. But it's all turned into much now.