As the language grows, so does the number of interactions between features. All the little corner cases start to get in the way of writing clean, useful code.
As the language grows, it also becomes harder for new programmers to learn it effectively, by which I mean not just knowing the syntax, but also understanding the idioms and being familiar with the libraries to make good use of the language in practice. For real world projects that make use of the advanced features, it therefore becomes harder to find staff who are sufficiently clued up in the areas that are in common use on any given project.
The more programming I've learned, the more I am convinced that effective programming languages are simple. This doesn't mean they can't also be powerful, but the programming model is clear and the interaction between different features is either systematic or non-existent. Nearly every widely successful industrial language achieved its success initially when it was a simple language. Many industrial failures -- in the sense that they continue to be used more because of various kinds of momentum than because of their technical merit as programming tools -- became so when they stopped being simple.