The reason is frequently that unreasonable requests are being made. Years ago I was working on a personal project that involved photographing friends in their home environments. I knew I would want to show this work in my portfolio and exhibit the work whenever possible. I had heard of model releases and knew somewhat about their importance. I was not super clear on all this, so when directed to an ASMP book for the proper wording, I simply copied what was there and used it. I added my name and the models and headed off on my project. These business details were issues I knew I should learn about, but the image making was what I loved and the excitement of having the first project that professors, fellow students and curators were showing genuine interest in was intoxicating.
After each sitting, I felt fantastic. I was excited about developing the film (yes, I am showing my age, hang with me though as the story is worth it) and proofing the work. My subjects seemed equally pleased and looked forward to the print I promised them. However, before I left them, I had to get out that model release and ask for a signature. It was awkward especially as I watched their facial expressions change the more they read the document. Now, these were fellow students and they ultimately did sign the release for me, however, that fear of asking stayed with me and negatively impacted my early business years in this area.
I realized many years later that I was simply asking for too much. Those cookie cutter commercial releases ask for a lot. They are necessary if you are planning to sell images as stock. They were not necessary for having in my portfolio or exhibiting in an art show. For the latter, a much less restrictive release is needed.
Why ask for permissions you currently do not need? It is always possible to go back and get a broader release from a model if a new buyer shows up on the scene. If a business wants a large commercial use that can demand a large price tag, you can (and should in my opinion) share some of that money with the model. Cash usually yields signed releases. The model (professional or not) likely just wants to be treated with respect. They want the reasonable compensation for the request being made. If the payment is a print, the Promo Release in the new ASMP Releases app is perfect. If the payment is real money, the ASMP Standard Release for assignments or the Getty Images Release accepted worldwide for stock photography licensing, also available in the ASMP app, are the ones to use.
It all boils down to common sense and not getting greedy. Focus on making amazing images and getting releases that allow you to get the work out in the world and noticed. Then get the big beefy release if and when that big sales opportunity happens.