I also kind of like That '80s Show. That '70s Show is still the better show, but That '80s Show had it's charm. And I watched that show BEFORE I had become really obsessed with the 1980s, though I had liked the '70s and '80s before then.
That 80's show had too many things going against it. It was far too early, for one thing. Had they waited just a few years when 80's style was relevant again, it might have got the audience it was looking for. Secondly, creating that show meant that That 70's Show was on a tight leash (the show lasted longer than the period of time it took place in. Remember, it was about the mid-70's). That 70's Show could never grow and expand into the early 80's (pretty much why they stopped showing the date on the license plate at the end of the show in the last 2 seasons), freezing characters in a year for 3 seasons. And lastly, the characters weren't half as fun and far more time sensitive than the cast of That 70's Show.
The overall appeal of 70's wasn't the fact that it took place in the 70's, but rather the fact that things didn't really change. Other than a handful of episodes that had a major time sensitive plot point, you almost could forget that it was the 70's if it wasn't the show's title. The characters rang true no matter what timeline they were in. That 80's Show was the opposite. Everything was time sensitive, the characters were all culled from 1980's tropes... they forgot what made the last show special and just did a bunch of 80's jokes.