Why they didnt stop the Delta viariant from spreading around the world?

So i have this same question i had when Covid started in China. Why didnt they do everything to stop it from travelling around the world? And back then, we had an excuse (lets say) that we didnt know much about it.

But what about now and this Delta viariant from India?
Why they let it spread to all the countries while they knew it was a more dangerous one than other variants?

Why didnt they act precautionary?
Is it because their only focus is on the vaccines and how to get everyone vaccinated?
Do they use this variant to scare the people who hesitate to get the vaccines?

I really dont get it.