An expansion doesn't have to mean a level cap increase, and a level cap increase is not an inherently good thing. Adding more content at the current level cap can quite often be a better option, instead of simply putting everyone on the level treadmill, even with new content.
Being Level 60 is not really all that much better then being lvl 50. Not as long as they add things to do at level 50.
Shoving RA-Upper Half in there was not a level cap increase, it's just readjusting the current leveling structure so they don't have to increase the cap above 50.
There is already little to do that is interesting or engaging once you've reached 50 and played their 'end game' stuff for a month or so. Making it 55 or 60 without anything additional things to 'do' would be pointless. I don't know what type of games some of you are playing, but a level cap increase ALWAYS comes with new content of some kind(usually PvP related). However since PvP in this game is so terrible they are actually talking about removing it from STO altogether I have no clue how Cryptic plans to keep the max level players playing going forward.
Maybe they'll just lift the max amount of ships you can have to 'unlimited' then announce a ZEN sale.
They have said that they want to introduce different gameplay with the upper admiral ranks. so if you get to 60 there has to be something different waiting for you, probably some kind control over multiple ship gameplay. there also needs to be a sizeable amount of content to get there, which they have not been able to produce before.
so that is going to take time to develop and there have been other things on the schedule first.
however, the ultimate goal of level cap increases is to give you ways to get stronger and have new powers. they can do that through the rep system.