Notosuchia has a long and storied past. They seem to have gotten by just fine with dinosaurs dominating the makeup of mid and late Mesozoic megafauna. Despite dinosaurs having more developed and specialized anatomy, notosuchians thrived with diversity of their own.
Most importantly, many of these are large, terrestrial, cursorial carnivores -- exactly the niche of the more established dinosaurs. This obvious competition does not seem to have fazed notosuchian diversity.
Notosuchians survived the K-Pg extinction event with Sebecosuchia. Together with the unrelated mekosuchines, long-legged terrestrial crocodylomorphs managed to eke out an existence for a few more tens of millions of years. However, they only thrived in areas with no significant mammalian carnivorous megafauna - pre-Interchange South America, Australia and Oceanic islands, mainly.
For some reason, the ecological advantages that allowed terrestrial crocodilians to coexist with dinosaurs and terror birds did not apply in the middle Miocene.
Since this era is also the one where large, competent mammalian predators had evolved, this implies that they were able to confine the notosuchians and mekosuchines to more specific areas and niches until they eventually went extinct.
Of course the climate of the Miocene was much different from that of the Mesozoic, I just don't know how that applies.
Is there something about mammals that made them harder to compete against even dinosaurs? Or is the bigger story more nuanced?