Why are lions the only social cats?
This is not a direct answer to your question, but I want to point out that your basic premise is partially incorrect. Other felines also form social groups. For instance, male cheetahs form coalitions (also see Cheetah outreach at http://www.cheetah.co.za/c_info.html), often for life, which generally makes them more successful in defending territories. Female feral cats also form social groups in some environments (see e.g. Natoli et al, 2007), where females sometimes co-rear kittens. There is even evidence that tigers share kills with related or unrelated individuals (see Wikipedia: Tiger for examples), and tiger home ranges can overlap in complex ways. On the other hand, lions (both males and females) are sometimes solitary, and this partially interacts with their living environment. 

My point is mainly that sociality (in general or in felines in particular) is not either/or for a particular species, and other felines also show social behaviours and sometimes form groups (so sociality should be viewed as a sliding scale). Granted, lions are definitely the feline with the strongest social behaviors, and they form the largest social groups. It is therefore interesting to think about why that is the case, and what group dynamics and tradeoffs that determine the social behaviours of lions.