They live in environments that change drastically throughout the day, like the droplets of water on moss. The moss will sometimes dry out, and to survive this they enter their tun state - think hibernation combined with freeze-drying yourself. They end up a shriveled little ball a fraction of their original size, and then expand and return to normal when the water comes back. They also can enter their tun state for things like temperature changes they'd otherwise die from. They don't seem to age much during this process and have lots of resistance to radiation in their tun state, which is a lot more useful when you're that small and radiation damaging a single cell could mean you lose an entire eye.

Different tardigrade species evolved in different harsh environments. E.g. where there are harsh droughts, or extreme colds.

So some species can e.g. survive the cold outer space by drying themselves up as a side effect.

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You aren't asking this about millions of species that did die out because they aren't resistant to much.

Its sort of like asking "why did THAT someone win the lottery last time?" - Millions upon millions of partakers, someone wins regularly.

PS: Not that there isn't a possible answer to this question, I'm just viewing this from an applied statistics angle.

Why? You assume there is a reason?

Oke, this is how evolution works.

Don't die and procreate. Every living being you see is what's left of billions of itterations of not getting dead before they procreated.

The tardigrade is no different.

However. Tardigrades have their size going for them. They are not particularly extra resilient by design but being small makes them extra hardy. Weird thing is that tardigrades are famous for it but most critters that size share those traits to a similar degree.