So, there are quite a few successful American companies in China. A lot of them are in the service and manufacturing industry, even the entertainment industry is relatively successful as long as artists don't wade into political issues.
The issue is technology companies, and really its a tit-for-tat battle. America refuses to let Huawei robust access to the American market on 'national security concerns' so China does the same to U.S technology companies. Part of it is letting domestic technology companies lead the way (i.e. infant industry argument) another is national security reasons. Apple is an interesting case. Apple's hardware business isn't at risk in China (China isn't going to ban Apple products). But, it's services (iCloud, iTunes, ext.) all must comply with Chinese laws. The court cases its losing are more a show of force than anything long lasting and basically a warning shot to Apple rather than anything more. Apple is actually a really successful company in China, despite what click-bait articles might suggest, any Anti-Apple protests are short-lived and have zero support from the vast majority. It's a way to show patriotism more than anti-americanism or anti-american company.
Uber was pretty successful in China and rivaled Didi pretty well. The trouble is that both companies aren't earning much money so consolidation is a step towards profitability. Even before this, Uber and Didi were technically illegal, but the CCP turned a blind eye for the most part. So, this isn't so much about Uber not being successful in China because it's an American company, more about consolidating and monopolizing the market rather than competition.
And, honestly, it wouldn't surprise me Apple were involved in these talks somehow. This could be a huge round-about way to get access to mapping data and self-driving car technology. It's not secret Uber and Didi are investing in self-driving cars, along with Baidu and other Chinese companies. Plus, remember that time Apple invested $1 Billion in Didi and then Didi invested $1 Billion Uber... That's just a coincidence, right?... It couldn't be Apple playing a long-con...could it? No... Never! Companies are honest, and straightforward and trustworthy!