Two prong appliances (lamps, vacuums, etc) fall into many types:
- Double insulated - Two protective barriers would have to fail to shock someone, very safe.
- Polarized - Preventing the ‘hot’ from ever touching any exposed surfaces, but still pose a risk to safety if there ever is a floating neutral.
- Unpolarized - The ‘hot’ has no restriction on the flow electricity, and metal parts can be energized.
With a three prong plug, you know the ‘older’ ways of wiring appliances, such as #3, are gone. This way you meet the safest condition for appliance users.
Also, some added features, three prong plugs seem to be more secure once fully inserted, so children have a harder time playing with them. Three prong plugs are cheaper so home owners will be more apt to fix broken ones sooner.
I didn’t cover all of the pluses, but these are enough for answering why?