Einstein began working on unified field theory at a time when the weak and strong force were perhaps not known I believe...but as the years proceeded and the weak and strong force were being outlined why was it he missed these as necessary components to complete his field theory. I guess what was known when is very difficult to ascertain?
You can unify some of the forces of nature without unifying others. That's what's happened historically. Maxwell unified electricity and magnetism. In the 1970s we unified the electromagnetic and electroweak forces. Einstein was trying to unify the electromagnetic and gravitational forces. The discovery of forces A and B doesn't imply that we shouldn't try to unify forces C and D.
Note also that Einstein was working on classical unified theories, but you aren't going to get a classical theory of the weak force because the W and Z are unstable.