One other important factor involved is not based on the severity of the disease, nor the targetability of it. It's cost related. Anything that threatens "the west" gets more attention than diseases in e.g. subsaharan Africa because there is more money to make in richer countries.

I work at a company that develops immuno therapies for (initially) late stage (insert type, I can't tell, sorry) cancer. Our antibodies could be designed against basically anything but we picked a target that's not too crowded on the market and affects enough people that we expect to earn back our research investments and make some profit to invest in new therapies.