The short answer is 'supply and demand' if there is insufficient demand the growers cannot afford to speculate. As in other businesses there has been some consolidation and shuffling of growers in the last several years and plant selection is perhaps a victim. This would be an excellent time of year to discuss with your growers what plants you would like to see in your garden center next season. Perhaps consider pre-ordering on a contract basis.

On the other hand it is possible that your favorites have been discontinued because they have been superceded by 'new and improved' selections. New isn't always better, but sometimes there truly are better plants out there, don't be afraid to try the newer selections.