There are multiple factors involved. The extremely high rate of spread of Omicron does a lot to bring it to the top. More subtle is the difference between imm

There are multiple factors involved. The extremely high rate of spread of Omicron does a lot to bring it to the top. More subtle is the difference between immunity generated by each. That is, it appears that vaccinations and post infection immunity from Delta are little help against Omicron infections, but post Omicron immunity is highly effective against Delta. This appears to be related primarily to differences in the spike protein.