I don't think "bloat" is the issue; as I'm sure you know, Rust programs only contain code for the features of the stdlib they use; if it had JSON support and it wasn't used, the linker would omit that code from the final binary. (Granted, that would make the linker work a little harder.)
More at issue is maintenance burden. Data formats come and go over time. In 10 or 20 years when JSON has faded to XML's level of relevance, the Rust stdlib team shouldn't have to continue maintaining a JSON parser.
I don't think "bloat" is the issue; as I'm sure you know, Rust programs only contain code for the features of the stdlib they use; if it had JSON support and it wasn't used, the linker would omit that code from the final binary. (Granted, that would make the linker work a little harder.)
More at issue is maintenance burden. Data formats come and go over time. In 10 or 20 years when JSON has faded to XML's level of relevance, the Rust stdlib team shouldn't have to continue maintaining a JSON parser.