They ignore too many AP4 heavy weapons. For most other races 3+ is really special hard to get armour. They're too cheap for dudes with such high saves, with AP5 bolters combined with the array of tactical options and now "special" options available to only marine books.
The idea at first was marines were expensive and most weapons reduced the save to 4+ (in 2nd edition) but after that marines have been a fundamentally broken part of the game.
They should be maybe 1 or 2 points cheaper, but have a 4+ save instead.
Then you'll see other races besides marines will be decent on the table.