Now that's an actual question. We are left with an assemblage of
numerous fossil carnivores from the Early to Late Cenozoic, and now we
only have one group of terrestrial, carnivorous placentals left, the
famed Carnivora. As for the mesonychids, their shearing mechanism was
obviously less effective than the carnassial structure of the creodonts
and carnivores, so their demise can be explained by the simple fact that
their dentition wasn't as effective as that of creodont's and carnivore's.

The creodonts, on the other hand, were equal in size and strength to
that of the carnivores, although some have claimed that they were
inferior in intelligence to that of the carnivora, there really, as of
now, is no evidence for that particular speculation whatsoever.

So we are right at the beginning again, why did all the eutherian
carnivores die out save for the Carnivora (excluding cetaceans). It
might have been due to climate change, because when the climate cooled
at the end of the Eocene the majority of the mesonychids died out, save
for /Mongolestes/, which died out 4 million years after, during the
Rupelian. The oxyaenidae, a family of creodonts that did not survive the
End Eocene extinction event, their extinction is attributable to
competition with the nimravids and climate change.

The Hyaenodontids, the Oxyaenid's sister family, continued to thrive
right up until the end of the Oligocene, with only four genera surviving
into the Miocene. Once the last creodont genus /Dissopsalis/ died out at
the close of the Miocene, this great radiation of eutherian carnivores
has finally disappeared, for all of eternity.