I think other teams are maybe scared of it being high risk (but high reward). Then again, every team and their dog play out from the back now and it is equally high risk.
I reckon Flick is just very good at coaching it. It does allow you to play very high and squeeze the space, and if you get it right, it means that one or two passes can get you in the opponent's final third, or even in on goal. Playing so high also allows you to get right on top of the opposing striker, and win so many long balls (partly the reason those are played is because all the lines are high so the Barca forwards are forcing the GK or CBs of the opponent to kick it long). This is probably why Barca are so direct now - the aggressively high line and press in midfield and attack allows them to be.
As for your question, Barcaman. I have thought about it and don't really have an answer...playing the 'offside trap' does feel like a thing from a bygone era. But it's good to see that Barcelona are doing it. I would urge caution though because there can be days where the timing is off and the opponent can get in. But Flick seems to have coached it so well that 95 percent of the time Inigo and Cubarsi, and the rest of the back four, will trigger it perfectly. He has clearly drilled them very very hard, as it is a key component of Flick's style and offers so many benefits further up the pitch (as illustrated by the previous couple of games).
But I really don't have an answer for why few teams do it. Maybe other teams will copy Barca now as they did with Guardiola's high pressing, playing out from the back and passing game. They'll be watching and thinking if Barca can do it, they can try it too.