The best explanation I have seen for this is that the plastic bag increases the distance between the read head and the magnetic strip. As the magnetic strip is used, small particles break off and become embedded elsewhere on the stripe. These have very little magnetic force, but enough that they throw off today's more sensitive readers, even though they're embedded in a larger area of stronger magnetism.
By increasing the distance between the read head and the mag stripe, even though it's just the thickness of the plastic bag, the smaller "noisy" bits impart a smaller field on the sensor than the larger areas of magnetic material surrounding them. It increases the signal to noise ratio by placing a low pass filter between the strip and the read head.
In other words, it's similar to putting on earmuffs so you can only hear the bass line of a piece of music, muffling the midrange, and almost muting the high range frequencies.