Softbank has stakes in Didi, Didi has stakes in Uber, Uber has stakes in Didi, with this Softbank got stakes in Uber.
Uber is mostly successful in North America and Southern Americas. Didi is obviously successful in China and will expand to Southern Americas soon.
Europe is not an important market for these companies. Too much regulations and good public transportation won't allow them to thrive.
Softbank probably wants Uber to be limited to North America and have its companies like 99 and Ola compete with Uber in their markets.
If Uber is limited to US and Canada and a few small markets here and there, then it makes sense to see its valuation go down like this.
Softbank is competing with Uber in a global scale using a network of companies it invested in, including Uber!