1. Moderna and Pfizer are difficult and expensive to manufacture.
2. We don't know how long their protection lasts. So far we know it's good for at least 6 months.
3. Covid keeps mutating (like the flu) which means that we'll probably need new booster shots every year to keep track.

All this makes it hard to keep the population vaccinated. But on the other hand if you only needed to take a series of pills *after* you get sick then it's a powerful tool to help those who can't (or won't) get vaccinated. You also only need to make enough to keep up with folks who are actually sick instead of needed to account for the entire population of the world.

I never heard of this treatment before today but if it works, then it could be great.