The assembly is there for performance. Other curves are implemented in constant time pure Go. (It's actually more robust than constant time C, because the Go compiler is less creative in "optimizing" things into branches.)
Heh, I used to joke that in Go it's safer because I know where the compiler team sits, but I think there's also a fundamental design reason: the Go compiler has speed as a primary goal. I don't know if that's still true, but there used to be a rule of thumb that a new optimization had to speed up the compiler enough to make up for its cost to be added. It's unlikely that unwinding bit masks into branches would be worth adding.
Anyway, we're not moving all cryptography to assembly anytime soon, and I'm positive that would not be a net increase in security, even if it would mitigate the hypothetical risk of compiler-introduced side-channels.
