This question admittedly contains a generalisation. There may well be traditional martial arts (especially particular schools/branches) that do seek to evolve and improve.
However, it seems fair to argue that if a martial art sincerely aims to develop and provide an effective range of fighting skills, then it would make sense for that martial art to incorporate aspects of other martial arts that contribute to or improve upon the style's existing body of knowledge.
Another way of looking at it is to ask why more traditional forms have not over time gradually moved more towards a contemporary multidisciplinary approach, such as those of MMA/Krav Maga. Even a very pragmatic style such as Goju-Ryu could benefit from incorporating the best elements of other styles, and many of the weapons elements of many traditions seem particularly redundant now.
Granted, there may be aspects of martial arts which are distinct from combat application and which are deemed important, such as tradition, identity and sport, but these would appear to be secondary to any serious martial art's aim of providing a effective skill set, especially as globalisation and information technology now facilitate awareness of alternate techniques so effectively.
Perhaps efficacy is simply not the primary driver any more. Perhaps business considerations come in to play. Perhaps this phenomenon is a result of the fact we now live in a much safer society; one in which martial arts are more important as a recreation than as a necessary survival tool. Perhaps pride is a factor.
Why are traditional martial arts apparently so reluctant to evolve?
EDIT: There have been some great responses to this question. It's been pointed out that my use of the word 'efficacy' is vague, because efficacy is so context-dependent. I agree. I was attempting to use 'efficacy' to describe 'real-fight' application; surely the original motivator behind the development of most early martial arts. There are clearly other motivators behind martial art design and practice and it is also clear that the original motivations give way to new ones as society changes and combat application fades relative to notions such as tradition, identity, sport, business and health. I look forward to more answers.