Because it was nerfed at the beginning of Season 5. Before Season 5 it was a great ship. I don't recall what all Cryptic did to it but they made it weak. I flew one from the day it came out to around Christmas 2011 when I gave up on it. Before season 5 it could do anything and I totally loved it. The Excelsior was GREAT in PVP, STF or just about any battle as a healer, fighter, support or whatever. Now it's a dead weight. I COULD make mine flyable and have it hold it's own in all the above situations but it would only hold it's own. Another good pilot in an simple escort would make it go boom in seconds. In the hands of an Instant Admiral, it would go boom in less than seconds.
Well, my Engineer still has one and i've been flying it lately in ESTFs. Its a lot of fun for me. I rotate ships every now and then to relieve the boredom.
I do remember when I was PvPing more and the Excelsior suddenly was not as good as it was, but really its vague in my mind as to what actually was done to make it less effective.