Because it doesn’t normally infect monkeys.  The original reason it was named monkeypox was because it was first discovered in captive monkeys in 1960(1).  Later research would show that monkeys in the wild CAN get infected and become sick from the virus but its very clear that they are catching it from some other animal.  Many (most?) viruses have at least some capacity to replicate in more than one species,  but most viruses and bacteria evolve to specialize on one (or a few) species to replicate in.  Somewhere out there is a “reservoir species” for mpox virus – the species that the virus has evolved to infect.  For instance, humans are the reservoir species for measles virus. Dogs are the reservoir for canine distemper virus,  which clearly started from a dog becoming infected with measles virus sometime in the 18th century but that has evolved to be a truly dog virus(2).  Measles virus itself clearly originated when a 6th Century human got a little to intimate with a cow and contracted rinderpest(3), a virus which used to infect cows until we humans vaccinated it off the planet.  The reservoir species for mpox virus is still unknown – despite the old name it isn’t monkeys and the smart bets are on some species of African rodent(4), and there are apparently quite a few of those (5).

So its mpox because this virus did not come from monkeys and perhaps they are hoping its a mouse.  What if its a species of rat or gerbil..rpox? gpox?

References

1 – PRIER JE, SAUER RM, MALSBERGER RG, SILLAMAN JM. Studies on a pox disease of monkeys. II. Isolation of the etiologic agent. Am J Vet Res. 1960 May;21:381-4. PMID: 14434871.

2 – Quintero-Gil et al 2019. Front Microbiol. Origin of Canine Distemper Virus: Consolidating Evidence to Understand Potential Zoonoses. (link) https://pubmed.ncbi.nlm.nih.gov/31555226/

3 – Dux et al. 2020 Science. Measles virus and rinderpest virus divergence dated to the sixth century BCE. (link) https://pubmed.ncbi.nlm.nih.gov/32554594/

4 – Atasoy MO, Naggar RFE, Rohaim MA, Munir M. Zoonotic and Zooanthroponotic Potential of Monkeypox. Adv Exp Med Biol. 2024;1451:75-90. doi: 10.1007/978-3-031-57165-7_5. PMID: 38801572.

5 – https://en.wikipedia.org/wiki/Category:Rodents_of_Africa