Mostly cost... the two pin things are cheap as dirt. Three prongs are harder to make so cost more.
As such, if the appliance does not need a ground, they go with the two pin job.
There is no dictated standard in North America that a plug on the end of the cord MUST have three pins regardless of insulation type.
Further, there are countless legacy outlets that have no hole to plug the ground pin into, even in relatively modern buildings. So adding the third pin can actually increase your product return numbers and reduce your market share.
The North American outlet system is rather old. Plugs never seem to fit right and it is really easy to touch the live pins while inserting and removing the plug from the outlet. They also contain no ground pin activated safety features.
On UK receptacles the ground pin opens a protection shutter on the power pins whether the ground is wired up or not. To reduce cost, some "double insulated" appliances have a dummy plastic ground pin on the plug.
Notice also on the plug that the live and neutral pins have an insulated length on them. The pins do not become live until the metal part is fully inside the receptacle.
Much safer and more reliable than this thing....
Electrically, three prong plugs (two power pins and ground) are only electrically required for devices that need to be earth grounded.
Many if not most small domestic appliances, tools, and other systems, are double insulated, that is, insulated in such a way that two faults must occur in the insulation before the user is exposed to lethal voltages. The fact that the customer can quite easily kill themselves while plugging it in, seems to be covered by an S.E.P. field. ( Somebody Else's Problem. )
The current mechanical nature of the aging North American plug itself, means that, unlike in other countries like the UK, there is NO safety benefit or socket feature that would add value to retaining the ground pin, or a dummy ground pin, when not needed electrically.
Costs therefore dictate that the smaller two pin plug be used when no grounding is required. Costs also dictate that the system is not going to get upgraded any time soon to include better, safer, plugs and outlets.