Einstein did not have the benefit of knowing that

a) There were other galaxies besides our own (Hubble found Andromeda in 1923)

b) These ‘other galaxies’ were receding from us – and from each other

Not knowing these experimental facts, Einstein postulated that there must be a non zero cosmological constant that keeps the Universe ‘static’ (not expanding or contracting).

Enter Friedmann

Friedmann, without having the experimental evidence himself, figured out that everything is receding from  everything else.

How? He made a simple , yet powerful assumption, while solving Einstein’s equations.

That the curvature that one sees is independent of where one is in the universe. That is – it is equally curved regardless of where you are. Based on this assumption, he showed that any such cosmological constant had to be ZERO.