The question is simple. I play a lot of pubgames and i have almost never seen a trilane. If you think that all pro teams form trilanes and it is the best strategy to get a lot of farm for your carry and win, why nobody tries the same in pubs?
One thing that's necessary to make tri-lanes work is that the supports need to know how to stack and pull neutral camps to be able to get XP and farm without sapping it from the carry, and handle the pulled waves so that they don't clear the neutrals too quickly and then mess up the lane equilibrium.
Furthermore, the carry has to expect and know how to deal with farming in this situation without any friendly creeps. If you pull a wave with a carry that's never seen this before, he's likely to get confused and die.
Furtherfurthermore, you need to make sure the solo offlane player is actually playing a hero that is viable for the role i.e. capable of securing some levels and farm alone against two or three heroes in his lane without feeding, and the list for that is relatively small compared to other roles.
It's all a level of knowledge and coordination that cannot be automatically assumed in pub play until you get to a certain level of experience/gameplay. If you just play a tri-lane the same way you would normally play a dual lane, except with two supports instead of one, all you're likely to end up with is three underlevelled heroes in one lane and one guy in the offlane that's either going to have to play too safely to get any farm and experience or die a lot.
Honestly I think it's a simple matter of lack of coordination.
The Pro Level DotA 2 teams practice together on a daily basis, they know their teammates, how they play, who can handle what, etc and it is a lot easier to coordinate a maneuver such as the Tri-Lane when you're confident in the rest of your team.
A Tri-Lane usually requires a pretty specific set of Heroes to be able to pull it off, and personally I don't usually see enough communication during picks for a PUB team to set something like that up. Half the games I play I consider myself lucky if we have a decent mid Hero...
There's a lot of details and play styles required to pull off an effective Tri-Lane and being matched with random players for a PUB match leaves too much to chance, so it's usually safer and easier to just go your standard 2-1-2 or 1-1-2-Jungle since that's what most people are familiar with and comfortable with.
Though I will say that a PUB team that could pull off a Tri-Lane would probably do some serious damage in Matchmaking. Good luck!