I am not an expert, but my understanding is that long GRBs are expected as the result of the collapse of very massive stars. These stars are short lived (few mi

I am not an expert, but my understanding is that long GRBs are expected as the result of the collapse of very massive stars. These stars are short lived (few million years) and very luminous. Metals (in astrophysics anything heavier than Helium) in the envelope of such a massive star would drive strong radiation-pressure driven mass losses (because of the increased opacity). Only very metal poor stars are able to remain sufficiently compact and massive at the onset of collapse to be able to trigger a GRB.