Probably one of the most widely-recognized reasons is skills. If a kid is still working on consistency on lv. 7/8 skills or if they are super advanced in one/two areas vs. the others, the coaches may move them to Xcel Diamond vs. DP lv. 7 or 8.
Also, there are a lot of gyms who use Xcel to help girls who may be more academically focused. If you are in an area with lots of AP/IB coursework, schools may be less willing to accommodate gym, so Xcel can have reduced hours and school-friendly practice times.
Age is also another reason I've seen. Xcel programs in my area tend to skew older/have a larger variety of ages than their DP equivalent. If a gym has a lv. 5 team and an Xcel Gold team, the lv. 5 team might be mostly 9-10 yo and Xcel gold could be 8-12 or 13. So gymnasts who start late might be a great fit for Xcel because they will likely find peers closer to their age in their level.