Because of numerous news reports and EPA tests: e.g. this one, this other one, and yet another.

I couldn't find updated reports for Delta, but the first two reports say things haven't gotten much better, so I'd assume the 12% of Delta planes where coliform was detected (per the last link above) is still true.

All that said, while it's not the cleanest water, the chances of you getting ill from drinking it, particularly if you're healthy, is low. And I still occasionally drink the coffee.

edit: I haven't really paid attention to see if the FAs are making coffee on DL Connections, since I usually fly AS/Horizon instead of DL Connection/SkyWest for my small hops, but this AA thread, seems to suggest that for ERJs the coffee isn't made onboard: http://www.flyertalk.com/forum/ameri...-day-long.html