It's just reached the end of it's operating life-time. I beleive the Tevatron exceeded it's original design plans re longevity, performance etc and like any piece of technology it's getting out-dated. It's just not worth running it any more: you're better off funding new. modern infrastructure that has a far broader/deeper research scope.

Fermilab itself is not closing down, just the Tevatron. The lab will continue to operate.