An alternative idea to the ones mentioned here already: Perhaps they simply didn't want to give away the fact that gallium doping is an idea worth holding onto.

An alternative idea to the ones mentioned here already: Perhaps they simply didn't want to give away the fact that gallium doping is an idea worth holding onto. By letting the patent timeout they save themselves tonnes of licensing money in the future, whilst also offering a reason for customers to upgrade. More cynically, they may have preferred the fact that the panels would degrade over time, driving sales. Now that the patent is expiring, they either dope with gallium or lose customers to their competitors.