I've played 4 string so long that any attempt I ever made at 5 was futile.
The thing is I would like to get one, but would have to spend a great deal of time getting used to it.
This may not happen to others, but I know how I adjust to new things, and if I'm gigging, learning the 5 will throw me off both ways; I wouldn't be good enough on the 5 to take on a gig, and my 4 string playing would be tripped up, at least for a while.
I know many don't have this issue, but I would. Also, I do things by habit, so much so that if I got used to 5, I wouldn't play my 4s anymor... and I don't want to start buying up new basses and getting rid of mine, which I would do rather than let them sit.
At the end of the day, I'm content playing 4; I know my way around on it, I can play and sing, which would take extra effort to get used to in changing... and even though I think 5s are great, they aren't for me... and I don't really like change...