Andy:
"Or maybe the brands weren't sponsors of the show and the show didn't want to give them free advertising."
It's just that showing brand names can be tricky, so sometimes it's avoided - though there is a lot of paid product placement going on nowadays.
Here's a reason it can be tricky - if a brand is being used by an evil character, the trademark owner may object - and may even have legal recourse if the depiction is such that it could tend to "bring the trademark into disrepute or public contempt" (this is very rare, but it's one of those things you don't bring on yourself if you can avoid it). The major exception is automobiles, which are difficult to disguise, so the tradition has sprung up that cars are kind of exempt. You also may have noticed over the years that when a show's credits say something like "Automobiles furnished by Ford Motor Company," all the bad guys drive GM and Chrysler products. It all came out in the wash, because when GM or Chrysler furnished vehicles, the baddies were quite likely to be driving Fords.
For a long time, product placement was frowned upon by the Federal Trade Commission, which is why shows and movies made in the later '70s through the later '90s often use fake trademarks. Roseanne did that a lot - look at the milk cartons, cereal boxes, etc. in the Conner kitchen sometime. They're almost all custom-designed and printed, with the sole exceptions of Shasta and Mello Yello pop. A very few trademarks were altered with stick-on labels, like on beer cans. It was a very impressive effort, because it costs a bloomin' fortune to do stuff like that. A graphics person has to design it, it has to be set up on the computer and then printed. And Roseanne was done before computers got so cheap and easy to use.
That has pretty much died out; it's now acceptable to do product placement again.