My guess, having only looked at the text of the law but not into any of the legislative history, is that it was for technical reasons. This is based on how they worded it. The text says it applies to "a television station, cable operator, or other multi-channel video programming distributor".

This suggests they were thinking of linear television. Some searching tells me that in fact this is how it was apparently interpreted, for when it was applied to cable TV it was not applied to on-demand cable programming. It was just applied to the regular cable channels.

With linear TV everything is prepared in advance. When they sell an ad slot they know what program they will be showing at the time. There's plenty of time to match the ad volume to the program volume, which I suspect in 2010 could not be reasonably automated.

With on-demand you don't know what programs the ad will be in until the program actually starts. You could potentially be showing that ad in thousands of different programs at approximately the same time. If the level adjustments could not reasonably be completely automated this may have been impractical.