Considering that it only took just over 30 mins for someone to reply that knows exactly why...
It had everything to do with the way Southwest flight ops defined a wet runway vs. the 2 other mainline operators (FL+DL) and the amount of seats that had to be blocked off constantly. City got whacked because SWA refused to adapt to an operation that was different than what they were used to doing over the last 4 decades. Nothing else, nothing more.