Nobody grows the older, and yes, even better varieties here in Thailand. They may have some trees, but any big farm has one goal in mind...the China market. And that means monthong.
It's not just the Chinese that want monthong - at least where I live, also most of the Thais seem to prefer the usual early-cut sweet cabbage crispy monthong over anything else that's available, and happily pay the higher price for it!
At the local Makro supermarket during this year's durian season, they had a lot of monthong the whole time, one counter section of puangmanee most of the time, often a few ganyao fruit, and some chanee once or twice. That probably somewhat indicates the local wholesale market and demand.
We've ordered rarer Thai varieties a few times from some "durian celebrity farm" that plants a lot of different stuff, but it has never been worth the price. Maybe the fruit have been good at the variety's original location due to age of trees, landscape, soil or cultivation practices (less spraying, cutting the fruit more ripe?) or whatever, but what we got usually felt to me like inferior monthong with less flesh. Won't order from that place again.
So commercially it clearly makes more sense for farmers just to grow monthong knowing they can sell everything wholesale for a good price, than to plan marketing/logistics for planting lot of varieties that ripen at different times and might not produce well or even taste that good in their location (or it rains a lot during the ripening one year and because of that customers think that this variety is bad and won't buy it again).
