The 737-900ER has been flying since 2007 (so 17 years ago) and never had this issue, whereas the 737-MAX 9 just started flying in 2018 (so 6 years ago). The evidence on the recovered door suggests it isn’t a design problem but rather an assembly problem. If indeed it is an assembly problem that existed 17 years ago too, then its much more likely that the older planes would have already had the failure the Max 9 experienced.
There’s also the practical impacts that this FAA notice brings. Boeing 737 comprises a huge amount of the worlds passenger airline capacity. The immediate requirement grounding Max 9 jets took a big chunk of air capacity offline. It takes time for maintenance crews to work through those. Making the same requirement for 737-NG (of which the 900ER is one) would mean those planes wouldn’t be flying either and sitting parked waiting for the maintenance crews to work through the Max 9 backlog.