Before Delta Rising, the PVP queues were reasonably active on both ground and space. I know it because I was there, and I did PVP many times. After Delta Rising the grind became, let's say, excessive because of the new spec tree, star ship trait system and the brutal gear upgrade system to name a few. Since then things have still never been the same.
Because if you want to compete in PVP at a reasonable level, you need at least Ultra Rare gear and various traits, unlocks and lots of planning and trial and error. And since STO is an increasingly more complex MMO than your average button-mashing MMO, the average noob player is too overwhelmed to persist. In Pre-DR days, I could grind a character to peak level in a relatively short period (2-4 weeks maybe), but now the grind is a million times worse even with the Phoenix lockbox and Admirality System.
Any new player starting the game NOW, with aspirations to max everything out and eventually do PVP is in for some impossible grind... unless they are willing to sacrifice their wallets, and maybe even their life savings at the Altar of Perfect World. Also adding to the fact that a completely new player doesn't even have like 20 farm toons and various account unlocks to start with in order to have a decent amount of dilithium and other comforts.
So assuming by some chance the noob player finally gets the necessary upgraded gear with all (fleet) trait unlocks, doffs etc. etc. after a year or two of grinding, then newbie steps into the arena and gets his TRIBBLE handed to him - either through some imbalance issue and/or because he lacked skill and/or proper planning. The player will then get extremely frustrated because all his time and effort to grind everything were for "nothing", and then rage-quitting becomes the first things on his mind.
Let's net forget also that STO is also the type of game that for some reason appeals mostly to role-players and 'fun-loving' casuals (of which I am neither). Adding so much grind since DR (and grind is not something these types of players enjoy) to the game was a bizarre decision to say the least.
So you see the average player will never persist going this far because too much is required. Only a few die-hards go for it, and those die hards are actually just veterans that have been playing the game since beta, who have all the benefits on their side. Cryptic could implement some sort of a better scaling and reward system like in other MMO's, but I'm not sure if they know how to do it. Personally I have accepted the decline, and am content to PVP with friends only. I still think it's a lot of fun, but it's obviously off-turning for most people because of the elite-level dedication required.