I find this question is backwards, we should never ask why not add a sigil, instead we should ask why should we add a sigil. In this case, there is insufficient motivation to add a new sigil into Rust.
This would allow anything that implements a trait to by truthy or falsy.
I'm not sure how that's significantly better. I don't like truthy/falsy values in general, so I really care if they are explicit or implicit.
The type system is turing complete, meaning that it can be just as complex as a built in operator.
Truthy/falsy values tend to lead to cryptic code, especially when used on more complex data types. But because they are so easy to use, people will try and shove them onto any type that fits (even if it doesn't have an obvious truthy/falsy value). So just by introducing them, truthy/falsy values tend to make code harder to read because they are so easy to use. (disclaimer: this is entirely my opinion, not based on very many hard facts, although it does seem to be a common sentiment).