I believe the primary objections are that it would require "it" to be a keyword (or at least a really special word) and it may interfere with a user using "it" as a variable or method name.
I'm not a fan of magic globals, either, and even though it doesn't start with $ this feels like the same sort of thing.
I'm not familiar with bug process here so let me know if I'm overstepping here. Since there's at least one other bug asking for exactly the same thing, I'm closing this as a duplicate.