From their blog post:
Finally, subjectively, C++ isn’t drawing in the crowds. We have never had a lot of C++ contributors. Over the 11 years fish used C++, only 17 people have at least 10 commits to the C++ code. We also don’t know a lot of people who would love to work on a C++ codebase in their free time.
Hard to tank when you don't have many to begin with. Rust is far nicer to new users to contribute to then old C++ code. Which can be seen in their github - in the last 24 months 16 people have contributed more then 10 commits. Which is during the conversion period - I dont expect that many of those to be C++ contributions. So rust does not seem to have hurt their contributions at all and in fact looks to have helped.
Syntax is in a large part what people are used to. Which is trivial to change by just using the thing for a while and getting used to the different syntax. But syntax is only part of a language. The tooling, documentation, error messages, and general feed back are all IMO much nicer in rust than C++. It is also easier to people new to programming or used to other languages to get into than C++ is, even including the syntax into that.
C++ was one of the first languages I learnt - and now after not using it for years I cannot stand its syntax.