The constitution gives congress the commerce power, but it specifically states the the congress may not tax exports. Why do you think the constitution placed that limit on congress?

Congress is not allowed to tax exports by US producers selling their products to foreign markets, because that would put a penalty on American producers and the US economy. In the early days when the Constitution was being written, the main concern would have been farm products exported to Europe. Today US exports include not only agricultural products but all sorts of manufactured products as well.