If we design a language with a large feature set, there's a combinatorial explosion in how these different features could interact, which means we're more likely to end up with situations where some language features interact together poorly. This distinction often increases the complexity of the language as there tends to be a duplication of language features and fairly arbitrary restrictions as to what code the compiler is able to run at compilation time. Unnecessary complexity in a language is bad for those learning the language, because it makes the language less intuitive and harder to learn.