Complex sentence: Many of the best cyclo-cross riders cross train in other cycling disciplines; however, cyclo-cross has reached such size and popularity that some racers are specialists, and many never race anything but cyclo-cross races.
Split into simpler sentences: Many of the best cyclo-cross riders cross train in other cycling disciplines. However, cyclo-cross has reached such a size and popularity that some racers are specialists, and many never race anything but cyclo-cross races.