There are two reasons why the PT basically prints money. One, the OT fans began the craze for the product and even though the product disappointed, we were almost obligated to see it through. I remember having to stand in line to buy my ticket a month before the movie came out. And then I had to stand in line a second time to actually watch the movie. With that much expectation going in, I almost deluded myself into thinking that TPM as an acceptably good movie.
The second reason was that TPM was very specifically engineered for consumption by children. I know it was Disney that began all of the Happy Meal toy and the product tie-ins, but TPM took it to another level. During the first three months of TPM's run, you could not go an entire commercial break without seeing a commercial related to TPM in some way.
If Lucas' goal was to made money, he accomplished that. But if his goal was to tell a great story, he failed miserably.