Bart's hydrops fetalis is lethal because –
`In the foetus excess, gamma globin chains form tetramers (1Ib Barts) with such high affinity for oxygen that they deliver almost no oxygen to tissues' - Robbins

Clinically this condition manifests as hydrops fetalis and fetus shows features similar to those of erythroblastosis fetalis → Severe pallor, Generalized edema, Massive hepatosplenomegaly