It's still a good item, especially since they just lowered it total cost by 297g, but Phage was once your staging point for building it (the first part that you built on the path to the Triforce), and the price increase on it when they re-tuned item costs really made it less attractive. These days, if you're sitting on 1500g, it's hard not to just buy a Giant's Belt and a Ruby Crystal, or Giant's Belt and Long Sword, or a Brutalizer and some Pots/Wards, etc.
Also, they nerfed the Slow and the MS on it, so that's another thing that makes other choices a bit more attractive. For instance, you could get a full Zephyr and a Giant's Belt for the cost of Triforce, and have, arguably, more useful stats.
I think another part of why it's less common these days is that people aren't going for the bundle items anymore, but focusing on just a couple stats at a time. Triforce gives utility, burst, and stats all rolled into one, but these days, I think that people just pick one or two of those things at a time and rely on their team, or their kit to pick up the slack.