Football, my friend, football. Pressure to win in basketball isn't nearly as great as in football, so the majority of their resources get focused on the grindiron.
It's fairly rare to have a consistently great football program and a consistently great basketball program. Texas and Ohio St are about as close as you can get in the last decade, but they are not what I would consider elite basketball programs (historically).
Look at the elite basketball programs (that have football), and most of the time their football is subpar. UNC, Duke, Kentucky, Kansas, UConn, Syracuse (for the last decade) ~ all great basketball programs with below average football programs.
I'd had to think long and hard about it, but I would probably say that Michigan State is the program that is fairly consistent in both football and basketball, but I wouldn't really consider them elite in football. Good, but not elite.