Why separate git repository for different versions?

[flamesea12]

Just wondering why not single git repository and use branches for different 
versions?
Instead:
haproxy-1.8.githaproxy-1.9.githaproxy-2.0.git
use git://git.haproxy.com/haproxy.gitanddevelop branch for current 
developmentand v2.0/v1.9 branches...