Cause it makes more sense not to count them at all rather than not counting them from some arbitrary date.
As for why they shouldn't count in gen dis at all, let me put it this way. The only tangible value of keeping track of post counts is to allow newbies to instantly tell who is a voice of authority within the community (avatars and badges being another). But if you only post on gen dis you probably aren't. Regardless of whatever your post count is, you probably don't pay that much attention to FS2.
You can always find your own post count fairly easily from last posts or your profile anyway.
