This is the explanation I have heard:
The TX750 was introduced in 1973 to compete in the horsepower race with other brands. It may have been one of those marketing things to clone some image from the TX750 to the TX650. But, the TX750 had problems, and became a drag on the Yamaha brand. So, the TX750 production was cancelled at the end of the 1974 model year, and the TX650 went back to the XS designation in 1975.