Sometimes failure is not an option.

Sometimes there is too much at stake.

Sometimes the cost is too great.

Sometimes you just are not fast enough.

(beware possible spoilers in comments)

Being the Deity of Time is tough work. You've got to protect the timestream, save ponies that tamper with time and get stuck in infinite loops.

There's also a lot of experimenting one can get done with the ability to reset the universe each time.

a lot of experimenting.

Point is, she's done a lot, for a LONG time. So, after her latest experiment goes very wrong, she decides to take a vacation.

Back to where she was once known as Twilight Sparkle.

Also, Discord does some things too.