It is odd that a value/virtue that plays such a central role in dramatic literature has played such a small role in philosophical writing. There are probably a number of reasons, but I think that a predilection for a certain kind of individualism is a major one. Others might include the fashionability of consequentialism, the idea that loyalty has more to do with sentiment than reason, as well as its proneness to corruption. The revival of interest in virtue/character as distinct from rules/principles has also created space for a renewed, if hesitant, interest in loyalty.