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.