Contrary to popular belief, it requires a fair bit of practice to get right, which is why you see hater GMs saying oh, yeah, it's so simple we could do it if we wanted, but then they try it in a game after one practice and it fails. The Eagles spent several years practicing it, so now they're that far behind.
They need to get some rugby coaches in to teach it. That's a super-basic rugby technique, which even I (as only a very casual rugby fan) can see most teams getting wrong.
A rugby scrum is highly regimented, it's not the optimal way for 3 (or 5 or 8) guys to push the other team back, it's the optimal way to do it given that they must be bound in a particular way.
A rugby ruck or maul is more freeform and maybe some of the techniques from those can be applied to NFL, but small differences in rules make a big difference there too.
On a different question, though, sure, the Eagles have a massive and strong QB who is perfect for this play, but other teams have huge guys playing other positions. Why not have a different quarterback for your QB sneak / tush push plays? Specialist players for niche situations is a trademark of the NFL compared to other professional sports, and this play doesn't rely on the element of surprise. You don't need to have your best player at passing the ball also be the strongest at breaking the line.
I was thinking of a maul, yeah. What I've spotted is NFL guys pushing with their hands on a teammate's back, instead of (like in rugby, or what the Eagles do) getting their shoulders against his ass or upper thighs, which gives more power and better leverage.
Good point about using different players, and I even think I've seen that a time or two. As for why not always do it, I'm only guessing, but there may be an inherent advantage in preserving the possibility that it won't be a "tush push" play (I mean, maybe not for the Eagles, because they're so good at it, but for other teams who aren't). Like, maybe it keeps the linebackers a step or two deeper and increases the likelihood of success; or, if they provoke a "tush push" defense that opens up a more-promising play then the QB is best placed to run the counter. I don't really know, though.
