I've only encountered this a few times, and I'm curious as to possible reasons. Why do some providers prefer not to go cowgirl?

Because, miraggio [gotta love that username],

Providers are no different than any other women out there - different strokes for different folks!

Possible reasons: just doesn't like it, clitoris is oversensitive, bladder can get 'bruised' from too much frontal action, for some reason can't orgasm that way, prefer doggie, prefer mish, doesn't like to stare down at ugly client [LOL], feels her breasts don't look their best from that angle, tired from having done cowgirl earlier in the day, cramp in the thighs, maybe has her period and doesn't want the penis to disrupt whatever stopper inserted into the vagina, doesn't like to do anything 'on command' - what else could there be?

Y'all tell me,
The Love Goddess

i don't like knowing i'm being stared at

i don't like looking down at anyone or anyone looking up at me (mental thing)

i don't like controlling the motion, because i don't know if the way i move will be the way he WANTS me to move

i don't like the inevitable eye contact (way too shy)

WOW! Incredible!

I hope I'd get some warning that I was about to book with someone who didn't want to look at me!
I can get that kind of "experience" from a couple cashiers I've known. I do not envy your clients.

Riding cowgirl takes more energy from the gal than doing mish or doggie where the guy does more of the "work".  She might be tired from earlier clients or you didn't pay her enough, in her opinion, to put in the extra effort. IMHO.

Nothing like a happy cowgirl

there is a tremendous difference between not being comfortable with direct eye contact (esp. extended direct eye contact), and simply not wanting to look at someone. perhaps you have never come across a shy or timid person, but we tend not to be comfortable staring at others and being stared at. a quick glance up and smile at his eyes, then snuggling my head beack against his chest, is more my natural style. fortunately for me, my clients seem to find this appealing.