I've been searching to find if there is a reason why JBIG support was 
not included in the OpenBSD port of libtiff.

All I've found is this message suggesting it could be enabled if there 
was a demand for it.

https://marc.info/?l=openbsd-ports&m=127761152201730&w=2

Other than lack of requests, is there any other reason JBIG hasn't been 
included in this port?

The HylaFAX+ port would benefit by having it included.