Dealers won't bother CPOing loaners because the value isn't there given then near $2k that it could cost to CPO. When buyers are only losing ~10,000 miles of warranty it just makes the vehicle too expensive (given a near-new selling price). When vehicles are older, further depreciated, and don't have much factory warranty left the cost of CPO is saleable.
Also, most or all of the loaners are registered but not titled, so they are sold as new from a BMW financing and incentives perspective. That could be a reason that they aren't eligible for CPO, but I'm less sure how that works.