It is one of Tax Law. By closing the NFA registry, the Congress has essentially rendered its own revenue making ability, moot.
I didn't think of it in that light Antipitas, they are "giving up" a taxable revenue stream. I know that if we had an open registry, every detachable magazine fed rifle I would buy would have the "select-fire" option box checked. The extra $200 to the gov added onto the price would be well worth being able to have that option available to me.