Because three of the justices are close to or over 80 and there have been rumors of retirement.
Via Daily Caller:
Texas Sen. Ted Cruz anticipates another vacancy on the U.S. Supreme Court will arise in a matter of months.
“
I think we’ll have another Supreme Court vacancy this summer,” Cruz said at the Conservative Political Action Conference Thursday. “If that happens, as much as the left is crazy now,
they will go full Armageddon meltdown because the next vacancy is where we’ll have the ability to act and restore basic constitution protections.”
http://dailycaller.com/2017/02/23/ted-cruz-says-another-scotus-vacancy-is-just-months-away/