The 2600 just sucked as a computer.
In fact just the opposite. Activision was one of the first third-party game manufacturers and Atari tried to sue them into the ground for it.
It's widely believed that the massive glut of 3rd party games (with effectively zero quality control) for the 2600 partially contributed to the video game crash of 83 [1].
It's also one of the reasons Nintendo learned from this mistake and enforced everything from limitations around the total number of games a company could produce per year, to the seal of approval, etc. on their Nintendo Entertainment System.
Also having grown up with the Atari 2600 - I don't know anybody who would have described it as home computer. It was a video game console first and foremost. Are you possibly thinking of the Atari ST line? [2]
The Atari 2600 was an immensely popular home computer for a decade(ish), but it didn't exactly spark the personal computer revolution. Why? Because it used the iphone software distribution model. You could only buy licensed software (in the form of cartridges) even though technically it was of course a programmable computer. So it was as open as an iphone.
All the actual progress happened on Apple ][, C64, the Radio Shack computers and later the IBM clones. Because, obviously, anyone could write and sell any software they wanted so the market growth went exponential.
A lesson to society, there.