Why? I'll tell you why.
Yesterday, I went over to the betta area of my local Petsmart. There was a fish, who was dead, and it looked like the cup hadn't been cleaned in a week. And you know how small those cups are and how frequently they need to be changed.
Secondly, I bought my first betta fish in YEARS recently. I week after I bought him, he died. We were going to my puppy's training class on Sunday so we thought we'd go ahead and grab one while we were there, which I soon named Comet. As of today, Comet is dead and I only had him 2 weeks.
Lastly, wondering why their prices are so cheap? Well that's how much they pay to maintain their betta cups.
Please take this into consideration. Although Petco is a little more expensive, I'd rather go there and get a betta that lasts 2 years like my very first one and not get one that will only last a week or two.