First of all, let us go through a little bit of history lesson. Valve picked up Turtle Rock Studios when it was still working on Left 4 Dead, and the team has gone indie since again, with this particular version of Francis being dated back to pre-acquisition days. Well, you can now pick him up from Steam Workshop to see action in Left 4 Dead 2. Valve did mention that they are unable to promise for other characters to be introduced, but will still try their darndest best (or words according to that effect).
Francis can be enabled by an update to Left 4 Dead 2, which happens to be primarily in support of modders. To underline that point, Valve did invite one prominent modder (his nickname happens to be Splinks, otherwise known as Zander to the family at Valve) to work in its offices for a few months just to help them develop better tools. Obviously, this particular move has its rewards, where some of the more useful updates include Starter Kits, Maya Files, and Tutorials. Those who are curious can always check out the Tutorial which should also provide enough information to address any particular issues that might just arise eventually, and get them fixed.