It's a good question. One of the outcomes of both a quota and an export tax—the purpose behind these is to restrict Canadian supply—would be to raise U.S. prices. Both the export tax and a quota will do it.
In both cases, the challenge is if you are a small producer and you don't have quota, you cannot access that U.S. market, so there's no benefit to you from that higher price. Under an export tax, if you can cover the cost of production, at least you can get some of that benefit. From my perspective at least, that's why there is a benefit to the tax, even for the smaller producer.