Because you suck? Kidding, kidding.
It happens to all of us, man. You can't force it.
I generally bum myself out when I realize I had some pretty groovy riffs about 10 years ago in my old band. I don't seem to have that same creativity anymore, as well as my influences changed, I have more going on in my life and not as much time to just focus on writing music.
Ahhhh the good old days of raging beers and being single and not a care in the world!!