Because when the vaccine was being introduced the delta variant wasn’t around or was just starting to appear. The vaccine does work as sold with the non-delta variants, which is why you pretty much never hear about them anymore. They change what they say because conditions change. Just like you change your sequence as the situations develops. You don’t stay on a south flow just because that’s what the forecast said if the winds end up favoring north. You change it as you get new information. Life is dynamic, things change. And the more people who get vaccinated, means the fewer who catch it to begin with, which means the spread slows, which means it’s less likely to mutate again.
