Mostly because there are very few uncrushable chokepoints to tanktrap up. Even when there are, mines are much more effective as they'll snare the tank.
I think also the build time is just too long to make it worth while when you include the vulnerability of units while building. If somebody tries to build them aggressively they rarely get up more than one before they start taking fire and dropping models. If you're building them safely behind the lines you'll likely end up causing more issues for your own units than theirs. A mine you can quickly plop down and move on.
Obviously you don't want traps to be too quick to build so that they get spammed around, but you would probably see them more if knocked down the build time a bit.