partly it's payment details: a lot of patent extenders roll out right when the generic goes to over-the-counter status--insurance pays for the new prescription drug, but won't cover the old one at all anymore.

the one relevant to me on a daily basis is zyrtec/xyzal -- xyzal is just zyrtec filtered for the correct chiral form, but the prescription is cheaper than the OTC for me thanks to my insurances' rules.