I've been asking myself about this a lot lately. The Hui, at least according to their tradition, claim to descend from foreign Muslims (Arab, Persian, and Central Asian) who settled in China and intermarried with the locals. Today, of course, the Hui are more related to the Han than anything else in terms of genetics, language, and cuisine, with the only key difference being their religious practices and the cultural differences that result from that (like the pork thing).
The Kaifeng Jews are very much in the same boat as the Hui. They originated as foreigners and slowly assimilated into the Han through intermarriage, but unlike the Hui they were not able to salvage their identity or their religion.
I think the key difference is that Islam is a proselytizing religion while Judaism isn't. No doubt, the Hui community grew not just due to intermarriage but also because of conversions. The Kaifeng Jews didn't really care about converting anyone.