I think the problem is the multiplayer experience. The handhelds, which PRO is based from, isn't optimized for multiplayer. Rather than players playing with each other (Guild wars, group quests, etc), players end up playing alongside each other. The experience is very much single player at its core; the multiplayer aspect is pretty much composed solely of chat, trading, and PvP. Single player games are fun until there is no longer a story to play through and no more quests to finish. There hasn't been any new content for a while, so players pretty much have nothing left to do. You'd think PvP would be fun, which brings me to my next point.
PvP is the supposed end game; the feature that should keep players playing even after finishing all the content available. This is true for the Pokemon handhelds and even other MMOs. In PRO, though, this feature is not prioritized. There are multiple battle system bugs along with uncoded moves and abilities. This discourages people who would otherwise stay for PvP from staying. This imo is a massive problem since it's one of the few aspects of the game where players can truly play with each other.
tldr; we need new content constantly and/or an improved end game experience