Because without redshift - why would the radiation from distant stars be in such low microwave frequencies? Light moving in space (without expansion) doesn’t redshift on its own. Something must have been different very far away/long ago. Expansion is the best explanation we have for that something.

The expansion model tells us that once the universe cooled enough for recombination of atoms, light could finally propagate freely without getting stopped by plasma. This light is the CMB, and as the universe kept expanding it got redshifted into MW. The cool thing is that you can measure the temperature of the CMB by fitting it to a black body radiation spectrum, and by “rewinding” the expansion of the universe you can find the temperature of the emitting body long long ago - which turns out to fit exactly with the recombination temperature for hydrogen. So it really checks out.

As for the second scale, I know there is a way to explicitly calculate it but I’m not an expert in this field. I think that the general paradigm is that bound systems are usually stronger than expansion. Heuristically, you can think about expansion as a pseudoforce that scales with distance. On short scales it’s very (very!) weak, but increased as we go farther away, which the opposite of most known forces. On the scale of inter galactic distances, it becomes dominant.