It is somewhat not very clear what exactly you are trying to ask (bacterial DNA?), but bacteria is very easy to transform with plasmids, grow up large populations of quickly, and select for certain traits (such as antibiotic resistance)....but many researchers also use eukaryotes as well.
Rapid replication, and a lot of familiarity. Beyond that, as Boom Boom said eukaryotes are not uncommon at all. I would add that some, such as strains of E. Coli, also are fully mapped, and have been for quite a while. This is a bit like wondering, "why fruit flies?", and the answer is generaly, it's not just them, but once they were the best understood and easiest to use.