Because it’s in the middle?

Well, yeah, but the middle of what?  A quick google reveals several wrong answers: the middle of the keyboard, the middle of the 8 C notes on a piano.

There are 88 notes on a standard piano keyboard, and middle C is the 40th note.  With an even number of keys, there is no true “middle”  — the hypothetical middle key would lie between E and F above middle C.  And with 8 C notes, there can be no “middle” there either.

In fact, it is in the middle of the two staves used for keyboard music: the grand staff.

You are now armed with knowledge that even many players with years of experience do not possess.  Don’t abuse it!

The Grand Staff is composed of one staff with a Treble Clef (or G Clef) on top, and another staff beneath it with the Bass Clef (or F Clef).  Why are they named as such?  For that answer, you should complete the Staff. Clefs, and Ledger Lines tutorial on the excellent music theory site created by Ricci Adams.