Slow on the uptake

I understand that certain inventions take time to develop.

Electricity
Indoor plumbing (the single greatest invention of all time)
Nuclear power
Electric pants

But I can't help but wonder why others took so long to come about?

For example, wheels weren't added to luggage until 1989. Why did it take so long for someone to think of this?

And what about the collapsible and removable seats in mini-vans? In today's vehicles, these seats can be removed with relative ease, but less...