The bigger development teams don't focus on a single distro, as their development base is probably diverse enough to demand a certain degree of platform compatibility.
However, many of the smaller development teams will focus almost exclusively on Ubuntu, as it is the distro they will most likely have.
I know, from my own limited experience, that I have only ever attempted to make my systems work on Ubuntu and just allowed others to push their changes into the main development if they have specific platform that they prefer to use.
The bigger development teams don't focus on a single distro, as their development base is probably diverse enough to demand a certain degree of platform compatibility.
However, many of the smaller development teams will focus almost exclusively on Ubuntu, as it is the distro they will most likely have.
I know, from my own limited experience, that I have only ever attempted to make my systems work on Ubuntu and just allowed others to push their changes into the main development if they have specific platform that they prefer to use.