To understand why these were so popular, you have to understand that microwaves seemed like witchcraft in the 1980s. They were still a new idea, not everyone had one, and they made heating food a quick and easy process. Microwaves were cool, and so was microwaveable food.
Some foods should never go in a microwave, though, and milkshakes have to be pretty close to the top of that list. Milkshakes should be cold. This was a product of its era, and it wasn't a good one!