Several possibilities (more food for thought than reasoned scientific theories [Linked Image] )

1. From personal observation, European 'backstab' type connections (I have personally only used Wagos, but I assume these are very similar?) seem to be 'tighter' than the North American equivalent.

Specifically, the Wagos I've played with seem to have much more resistance to 'conductor rotation' than the backstab outlets in the States I also played with.

2. Redsy has a point I was previously unaware of but seems to make a lot of sense, that they're rated for 'terminating' only, not feed-thru, which seems to be where a lot of the problems lie.

3. Due to construction differences, NEMA style outlets seem to get a lot warmer at rated load than European ones, this could possibly affect the spring in a negative way.

4. Often the failure in North American devices happens over a long period of time, rather than being seen instantly. The electrical contractor who goes to investigate has only the owner's word that the outlet hasn't been overloaded by the addition of multitaps, etc. (And that owner may not know what a previous owner has done, and so forth)