It really doesn't. Anyone can grow top-quality marijuana, and it will be even better grown outdoors in full sun.
No, it won't. The sun doesn't shine for 24 hours, which is the optimum light level for young plants, and even in the middle of summer the sun rarely shines for 18 hours uninterrupted without cloud cover or wind and rain that knock trichromes from the plant, nor will plants in a grow room be eaten by wildlife etc . I'm not saying that you can't grow good weed outdoors, but indoors is much easier because you have optimal conditions that are next to impossible to recreate in the wild. Also, outdoor crops are time and labour intensive, they generally take twice as long to finish as an indoor crop (you can't dictate how many hours the sun shines for in the real world, you can in a grow room, and the plant reacts to day lengths) and require a lot of work every year to prepare the soil (because no matter how good the genes, any plant will succeed or fail on the basis of soil quality. Also, when growing under lights, it is possible to target the plant with particular light spectrum's that make particular growth period much more efficient, another thing you can't do with the sun.
But, I digress, this is all getting a long way from the original point. There will always be a market for relatively expensive weed, it is a myth to suggest that once legalised that everyone will start growing their own and there should be just as much scope for tax revenue from pot as there is from other legal intoxicants. Take Volstead for instance, a large reason for repeal was to get excise revenue from the sale of alcohol, and this was a time when "alky cookers" were a dime a dozen, entire inner-city neighbourhoods were converted into distilleries to meet the Prohibition market. And even after Volstead was repealed the illegal hooch continued to flow, for some time, but eventually the market lost it's appeal and the bootleggers abandoned it to legal distillers who could operate more efficiently and quench America's thirst at much more competitive prices. The same thing would eventually happen in a legal pot market, sure people could continue to grow their own, but as an abundance of relatively competitively priced, high quality product, would make all the time and labour that goes into growing high quality weed an unattractive option to just buying prepackeged, government taxed product, just like we do every day at liquor stores.
Tobacco requires much, much, more room to grow than marijuana. Consider that an ounce of tobacco lasts a typical smoker 1 day, whereas an ounce of marijuana lasts a typical pot smoker for months. Which means you have to grow 60 times the amount of tobacco, so you'll need that much more space to do so. And most people don't have that much space. And I'm not even considering the far more complicated curing process for tobacco, which also requires lots of space.
All you need to grow a years supply of marijuana is 30ft2 of garden space.
I dunno about anyone else, but an oz of tobacco last about the same time as an oz of weed. An oz of tobacco would last me a week, at least, and when i'm smoking weed heavily an oz would be lucky to last a week and a half, maybe two weeks if I ration it.
ETA: Actually, I'm thinking of a 2-oz pouch of tobacco, my bad, but I still couldn't smoke an oz of tobacco in a day, that would make me sick as a dog.