There are multiple universes in Trek, multiple timelines, and a dozen different ways to go between any of them, from red matter (as Set Harth
mentioned) to slingshots, to warp engine malfunctions, to transporter malfunctions, to the Guardian of Forever, to the device in "All Our Yesterdays", to Q, to anomalies, to what-have-you.
Who's to say each one of those methods produces the exact same kind of dimensional or time travel? We have an N = 1 for red matter, and it apparently creates time travel that causes an alternate reality to occur independent of the one that was original. People wanted original ideas from Abrams, et al. Hey, there's an original idea.