The drug was repatented with the delivery mechanism, as soon as the original patent expired.

It's the usual practice to keep novel delivery system/drug combinations off the market until the original drug patent expires, thereby extending the profitability of the patent, at the cost of users suffering medicine that is consequently decades out of date.

I think a similar issue applies with Epipen. There is an alternative, Adrenaclick (still expensive but not quite as bad), but because the delivery mechanism is different the prescription needs to be written differently.

Transdermal delivery unfortunately seems to be underexplored and very expensive in general. Transdermal estradiol is another one that is much more expensive than the pill form but is better in terms of safety. Generic patches available outside the US are less expensive (than patches in the US, still quite a lot more expensive than pill form). I'm not sure if generic patches are completely unavailable in the US or if they are just also quite expensive.

Melatonin is another case where transdermal delivery can be helpful, but it is difficult to find and fairly expensive (not quite as bad, though).

Beyond delivery mechanisms, drug companies also create modified forms of a drug (prodrug or metabolite or slightly different but similar substance) that have small advantages over the previous version. Again, these can't be substituted and doctors need to know to write the prescription for the form available as a generic. Best for the drug companies is when the origional form is pulled from the market by the FDA right after the patent expires, as happened with the antihistamine Seldane/terfenadine (replaced by Allegra/fexofenadine, a metabolite).

In other cases where out of patent drugs are not available as generics it is because the market is small and the current manufacturer can instantly drop their price to match if a generic appears, making it likely that anyone who attempts to make a generic will loose money.