Measles is the most virulent common infectious disease, clocking in at R0 ~ 12 to 18. For comparison, Covid's is 2 to 6 and chickenpox 10 to 12.
Only about 1 or 2 out of a 1,000 measles infections results in death [2]. But something like 1 in 4 results in hospitalization. We don't have that hospital capacity.
[1] https://en.wikipedia.org/wiki/Basic_reproduction_number
[2] https://www.cdc.gov/measles/symptoms/complications.html?CDC_...