I think that it has mostly to do with the co-specific agression of the species that makes them "under-rated". IMO if they gat along together, you would see multitudes of largeer tanks with massive schools of these beautiful fish.
IMO the problem is easy. You cannot keep these fish in schools. Even in a community tank setup, the Betta can only have tank mates NON-BETTA! I think that the inability to keep another member of the species housed together is the most major turn off going. If I were to be "Homed" I certainly would NOT want solitary confinement. It would be a little better to be confined with dogs and cats, but I still could not live that life! Confine me with a bunch of buddies in a massive party, and life might possibly be doable. Confine me with Sara Michelle Gellar, and I might not know that I was even confined for years!
I know I felt guilty as hell NOT giving my pet others of it's own species to habitate! I understood the less than zero chance of two males homed together, but I have see nearly as much agression between two females and to the death!!! I have tried close to a dozen times to home a M/F combination of Bettas in both a Betta only tank as well as part of a community of different peaceful species. Death has been the result without exception! I am utterly shocked that this species exists. How the F' do they mate in the wild?
I had a male Betta kill several Females, so I tried a different (More Normal?) Male and got exactly the same result, until one day it was the female that survived the male. That female has now killed four males (one at a time!). I have yet to ever see anything like a behavior that would propigate the species. They seem to kill (NO APPARANT MOTIVE!). I have even had this Female kill a male that made a bubble nest?
Conclusion. I feel guilty and will NOT keep these fish in one of thos half gallon pieces of crap. That is not fair to the fish, even though pet chains keep them in cups with less than a pint of water! I think as a fishkeeper gains experience (I am a newbie @ ~ the 2 year mark in fishkeeping) we revolt against the keeping of a fish in these deplorable conditions. We also have to face the extreme challenge of co-specific homing of the Betta as near impossible after a time. This leaves us only one option in keeping the Betta, and that is to supply it with it's own species tank with a minimum of 5 gallons and preferably 10 gallons and up!
I have utter disdain for solitary species tanks and find them disgustingly depressing regardless of how well planted, aquascaped and clean the setup may be! This species is evidently viable and does somehow mate in the wild. There must be something that we are sorely lacking in the area of the Betta hobby. There by definition (LIFE) has to be a way to peacefully keep members of this species together, it is simply not a matter of common public knowlege in the hobby at this point.
Next year, we may find out that the tannins from the leaves of the rumpatrumpa tree in sufficient quantities in water in their natural habitat make the Betta go into a "Grateful Dead Show" mode and make them want to get naked, dance and partake in wild fish orgies! I will have a 55G full of them at that time. At this time I am maxed out at three bettas (Two males and the Femmy Fatale) each in there own two gallon Betts tanks (The largest Petsmart had for them!!!).
I know, and YES I do feel guilty about them suffering in that tiny of spaces, but I am running well over twenty tanks and do not have the space for each Betta to have a 10G to itself. The Pygmy everglades sunfish is one of the most colorful fish just as the Betta. You can keep a decent sized school of them in a 10G tanks for life (10 years double the Betta!)