Because trinketing a nova to get renova'd works. Pet nova, trink, charge, reg nova, trapped, free after a cpl sec, hit with talented cone (if you even make it), stuck again. The only diff a 2 min trink makes vs a 5 min trink is uses in a game (obviously). It's not going to guarantee something such as a 1v1 simply because they don't usually last that long. Sure more uses is good and all, but there's still the downtime where the added dmg and sprint can prove to be just as useful.
Trinket a trap just to get scattered, trinket a nova to be renova'd, trink a blind to get kidney'd or cheap shot, blahblahblah. A 2 min pvp trink is nice, but I'd say if you're going for straight dmg the extra dps you can get in between being kited or w/e would prove more valuable. Plus the fact that mobility is limited and even if you get out of one of those there isn't the guarantee you won't be slowed, and that sprint could be the game breaker, life saver, etc.
My opinion of course, and I'm no pro (especially in twink brackets) with all the scenarios and this and that. Just figured I'd explain my reasoning behind it.