You're being downvoted, but that's actually a good question. An explanation that fits the fact is antigenic drift. A variant that's different but not necessarily that much worse will outcompete another variant in which the population has built up significant immunity. The difference in fitness doesn't need to be that large.

If the virus truly had a massively larger R0 value, as is asserted in TFA, then you'd expect it to catch fire extremely quickly as soon as it starts showing up, and everywhere it is found. Ron Fouchier (see TWiV 777 as cited elsethread) certainly believes that these huge R0 values are not plausible, and that antigenic drift fits the observable facts better.