Two reasons: (a) way too much competition between DL and CO in the NY Metro market including TATL service (CO had been running ads taking shots at DL for a while), and (b) DL/NW is so dominant within SkyTeam-US, that CO really didn't have a choice. It will be a strong TATL united states flagged carrier partner for UA, and will pick up access to UA's strong pacific routes. Plus, both UA and US are relatively weak in NY, so it was a big win for them.

As a NY-area person, it was nice knowing you always had a fallback to take a non-stop pretty much anywhere from EWR.

As to lounge access, Amex Platinum still gets you to DL/NW/AA and CO.