Because some people basically go batshit crazy when they find out and start getting you super-strongly gendered stuff, and you want to put off having to deal with that crap for as long as possible is one reason. For example, if you have a baby shower _before_ the birth (yes, ok, you're having a baby shower to start with) and you haven't told people the gender, there's only so much bad they can do along those lines...

Interesting perspective, thanks!

No problem. :)

Note that my experiences with this are all in the US. Now that I think about it, in cultures where there is more pressure to have kids of the "right" gender, there are other really good reasons to not tell people the gender even if you know...