Welcome @debbiev311, You are not alone, no one likes to be on prednisone. I don't have any medical training but I don't think they are able to determine what the exact cause for PMR is. Here's some info on the topic.

"The cause of polymyalgia rheumatica is unknown, but a combination of genetic and environmental factors is thought to be responsible. Polymyalgia rheumatica is an age-related condition. Most people diagnosed with it are over 65, and it's very rare in people younger than 50. It's also more common in women than men."
-- Polymyalgia rheumatica - NHS:
https://www.nhs.uk/conditions/polymyalgia-rheumatica/
I saw that you also asked a question in another discussion, is PMR contagious. I don't think it is contagious but I did see an older reference from 2012 about a married couple with both having PMR and thought to be from environmental causes.
-- Polymyalgia rheumatica in a married couple:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3430115/
You may want to scan the different discussions in the PMR Support Group to see if any of them might be helpful for learning more from other members experiences. Here's a link to the discussions in the PMR Group - https://connect.mayoclinic.org/group/polymyalgia-rheumatica-pmr/.

Have you discussed your concerns with the rheumatologist to see if there may be some alternative steroid sparing treatments?