I have 51 reputation on one of the Stack Exchange sites. Because of this, why can't I chat in rooms?
We have three chat servers - one serves this site, so you need 20 reputation here in this site, in order to chat on http://chat.meta.stackexchange.com.
The second serves Stack Overflow - so you need 20 rep there to chat on https://chat.stackoverflow.com.
The last one serves the rest of the network, including Area51. You need 20 rep on any of the other sites in order to chat on https://chat.stackexchange.com.