Because Nf6 is no longer pinned, Black threatens to win the e4-pawn.
The attempt to defend the e4-pawn, e.g. by 5.f3?! or 5.Bd3?!, will still lose the pawn, because White's Bg5 is not defended, so
Black can simply play 5...Nxe4! and win a pawn (6. Bxe7 Nxc3 7. Bxd8 Nxd1 8. Bxc7 Nxb2 -/+).
That's why White either plays the e4-pawn to a safe square (Main Line - 5.e5), or eliminates an attacker of e4 (5.Bxf6). A safe alternative is a transposition to the Exchange Variation (5.exd5).