It’s not so much about nuclear envelope and more about ends. DNA polymerase (an enzyme that builds new DNA) cannot copy the whole end - there are a few bases that should be at the end but cannot be added. Eukaryota deal with it by a complex mechanism (they have telomeres), but it allows for multiple chromosomes and therefore larger genomes. Bacteria have a circular genome instead, a circle doesn’t have an end, so they can copy as much as they need.

BTW, mitochondria and plastids, being former bacteria, also have circular genomes.