[algon33]

Because it has Omicron's increased ability to avoid people's immunity. That would result in higher infectivity than just a copy of the wuhan strain, with near the same severity. Note that earlier Sars viruses had even higher severity than Sars Covid 2 (Wuhan strain), but weren't as infective and killed far fewer people. Which implies that a slight reduction in severity for a gain in infectivity is not a worthwhile tradeoff.

[baja_blast]

Exactly, for example Ebola has a mortality rate of around 50% but has only killed around 11 thousand people, SARS-CoV2 has less than 1% can has killed 20 million or more. Wild viruses like SARS1 and MERS are poorly adapted towards humans when the spillover happened making it possible to be contained. But SARS2 a which is a Sarbecovirus a family of gastrointestinal viruses some how was able to bind towards human airways with a binding affinity 20 times that for humans than for bats, all while leaving no traceable trail of mutations that researchers could trace back to the intermediate host.

For context with SARS1 and MERS researchers were able to find the spill over animal within a few months. But after 3+ years we have yet to find an intermediate animal host. Also before anyone says "It took years to find the source for SARS1", but that is for the original bat virus, the intermediate animal where the cross over to humans happened was found within months. Additionally SARS1 and MERS had a rapid period of mutations as it adapted towards humans which allowed researchers to trace back to the source.

[Izkata]

You're confusing "severity" and "infectivity". Increased infectivity is not increased severity. The evidence they have shows reduced severity and an unknown (but theorized increased) infectivity.