Because always allocating them on the stack costs zero cycles in the typical case, while alloca costs more than zero cycles in the typical case. And assuming that "struct pollfd" isn't big, and the function isn't very recursive, there's no practical downside to wasting a little stack space for the life of the function.
(Of course, he could get rid of "bool ufds_malloc" and just see if his pointer is NULL before calling free(); or not even bother checking, since free(NULL) is defined to be a no-op.)
(Of course, he could get rid of "bool ufds_malloc" and just see if his pointer is NULL before calling free(); or not even bother checking, since free(NULL) is defined to be a no-op.)