The purpose of tie-in novels is to be a supplement of the property it's based on. Now that there's new Star Trek shows in production, it makes more sense to focus on publishing tie-in novels related to them that fans of the show can read then it does to focus on developed continuities based on shows that ended twenty years ago. TOS novels will still be around because they've always been the best selling Trek novels, so it would be silly to turn away that source of revenue.
Although, this year we've gotten/will be getting both a novel set during the TNG television era and a novel set during the DS9 television era. Maybe Pocket is testing the waters to see how viable those series still are during the current era of on-going new series.