Mr. Tabone,
Allow me to explain why I believe TKD has evolved over the years while TSD has not:
50 years ago, when it originally developed, TKD looked very similar to Japanese karate. Indeed, TKD and TSD originally were quite similar. While it did have some of the characteristic kicking techniques that we associate with TKD today, for the most part it retained the low stances, shorter karate-style kicks, Shotokan forms, and overall Japanese approach to execution of technique. In these aspects it was very similar to TSD. However, over time, the TKD powers that be made a purposeful decision to move TKD away from its Japanese influences and towards a more Koreanized style of martial art (emphasis mine). This included: characteristic Korean-style kicking, higher stances, discarding Shotokan and Chinese forms in favor of Korean-developed forms that have changed as need be, and modern TKD free fighting. It is awfully hard to be proud to practice a Korean martial art when you are still using Japanese forms and techniques. It is the TKD students who popularized long kicking and waist action.
On the other hand, TSD still retains a definite Japanese influence. By this I mean karate-style technique, short snappy kicking, and Japanese/Chinese originated forms. To me, watching a TSD practitioner is like watching a karate student. The way they execute technique is very similar. In fact, I've seen one famous Tae Kwon Do history site describe TSD as "simply Japanese karate as practiced in Korea." As one example, Chuck Norris, a TSD student, struck me as acting more like a karate student in how he did his techniques.
So in closing, I do feel that TKD has changed and evolved over the years: adding,taking away, and changing how it does technique as needed. TSD, on the other hand, still reminds me very much of Japanese karate and, to a certain extent, kung fu.