The chance of there being a thunder yellow is less than that of catching a shiny. If they did it would open up the, what about water blue arguments. Also, they wouldn't port it to the DS because they would loose platform linear perspective since fire red and leaf green are on GBA. There would be no reason to travel to orre since the only pokemon you can catch are at hot spots and you can already catch those elsewhere.
If there was a thunder yellow, It would be a similar upgrade to that of fire red, leaf green because, if it wasn't, then it wouldn't be like the original yellow.
Also, letting you travel to the other areas would make it unnecesary to buy the other pokemon games, which, would make nintendo lose money in sales and would be a very bad business venture.