If I had to ask myself, I'd say it's because domains that aren't .com very desirable, so it's possible that a decent .com domain won't be registered with other extensions. But, if I knew for sure, I wouldn't be here to ask the experts.
HealingDaily.com is also available as .org, .info, and many others, but the .com still sold for $20,500, and it's the same format as mine.