It was because Tesla would receive the full EV credit amount in California, which has to be paid by other car manufactures if their vehicles don't meet emissions guidelines.

California appears to be phasing out that requirement (ability to swap the pack), which is why battery swap stations are no longer being pursued aggressively.

http://articles.latimes.com/2013/may/05/business/la-fi-elect...

http://gas2.org/2014/07/02/ev-battery-swap-stations-may-be-a...