Even though, IMO, they are beautiful as youngsters, they go through a period of a brownish-grey color, and in reality, it often takes them 5 years to develop their adult coloration.
I suppose that is one reason they are less popular. Another is that you can't tell at all from their baby coloration what they will look like as adults. I've seen some fairly unattractive, muddy snakes come from a gorgeous set of parents.
Therefore, its a crap-shoot if you are looking for a nice pair to breed. Also, almost no one sells adult bairdi. They are a well-kept secret that few want to spread around.
The price is fairly low, the snakes are fairly calm, and their colors are not bright and sparkly.
To go one farther, the amels I've seen were far short of spectacular.
Those could be some of the reasons. However, like you said, when you own one, they move to the top of your list.
Some vendors-Vivid Reptiles, Gregg's Reptile Basement, VMS, and JDTails
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...and I think to myself, "What a wonderful world."