Hi betta_246,
I'm so sorry to read about you losses.
Since bettas are totally raised in captivity and jarred up at an early age, I don't think they ever have the chance to develop strong immune systems and resistance to disease like wild caught fish do. They are difficult to keep successfully and, all things considered, relatively few people can do it for any length of time.
Columnaris and finrot are bacterial diseases. Potentially harmful bacteria live in all tanks but strong fish are able to resist them and stay healthy. Bacterial disease can develop when a fish is exposed to unhealthy tank conditions and does not have the immunity to resist it. Any form of stress can lead to lowered immune systems and make the fish more apt to become sick even if some other fish in the tank remain healthy. Some common stress factors include over feeding, high nitrite, nitrate, or ammonia, aggressive tankmates, overcrowding, temperature fluctuations, or low oxygen level in the water. To make matters worse, once a fish becomes infected, it's not unusual for a secondary infection to set in. Some bacterial diseases like columnaris move quickly and by the time they are discovered it could be too late to cure a delicate fish.
No matter how clean and cycled a tank is, bettas have gone through bad conditions before they reach you and might have had setbacks you can never know about. Unless you buy them from a good breeder, and take possession in person, there's always a risk.
Good luck with your future purchases. With your knowledge of fishkeeping and the experience you gained from keeping these bettas, you will be more successful in the future.