Because it smells of Ticketus all over again.
Having been late to the realisation party over the conning, they don't want to be late again, even though they are.
The staunch berz feel the same about the Season tickets as they do about the keys to Ibrox. Problem is, as you said, it is their main source of income (except for a share issue perhaps , or the , both of which kill them).
Spending the Seasons cash when it is in = fine , mortgaging them again is not.