Older Bo's

All,

I was recently perusing through some of the newer Trade-A-Plane listings,
and I saw tons of old Bonanzas.
Within the last 3 days of listings, there were probably 10 or more.

From previous threads in this newsgroup, I have read that the older Bo's are
expensive to maintain, typically due to the high cost on parts.
So, I was wondering why the parts are so expensive and/or hard to come by if
there are so many planes out there?

Does someone have an older Bonanza who could comment on this?

Adam