It's difficult for vertebrates to aquire/evolve enucleated cells, that's why they are very rare previous to the mammalian adaptation. Salamanders have evolved enucleated cells. Research suggests that it may be due to a salamanders big genome and big cell nucleus, compared to the salamanders small body size.
Mammals may have acquired the trait because they evolved from small animals with miniaturized capillaries and blood cells and large genomes, and the advantage was kept afterwards. Large genomes inhibit flow and flex of nucleated blood cells.
The smallest bird genomes are found in the most miniturized clades of birds at 1 Gb. Mammals of a similar size can have carry 2-3 Gb of chromosomes.
Lizards have lower energy than mammals.
Fish are most active in highly oxygenated water, i.e. salmon, which allows them to be very energetic, water is a more efficient breathing medium than air, hence the small size of gills. fish and humans both contain 6-7 percent blood by volume.
Birds have smaller haemoglobin including the nucleus, and have a different kind of capillary size and structure with advantages and disadvantages, and lighter hollow bones with less marrow. Birds didn't radiate to new shapes as well as mammals did.
Enucleate cells are fairly rare in the plant and animal world, so they perhaps have a hidden expense. Maybe it's not just a factor of shedding the nucleus, and in fact there is a cost of adapting the primary circulation system's immunity against paramecia and pathogens without a nucleus. various viruses can take advantage of enucleate cells.
Perhaps that is the reason why birds did not evolve backwards into dinosaurs. some factor made their genome comparatively small. It could be the lack of enucleated bloodcells that forced them to shed genes as an alternative. Perhaps that's why they lost the dinosaur genes which held code for quadrupeds, arms, jaws, horns and teeth and the things that birds don't have.