I notice with some small appliances, made of metal, will have a three prong grounded cord, but others also metal, do not. Is there reason for this? Usually the grounded cords will be thicker, also.

I know with tools like Black and Decker drills, sometimes there's a double insulated symbol which means it doesn't need ground.

Also sometimes commercial versions of things will have grounded cord even if they're plastic, such as fans.

In the 70s, a lot of fans made of metal, started having grounded cords also, even if they didn't prior.

I know with computers and surge protectors they have to have it, due to the static or surge discharge.

Modern safety requirements come down to : You must have 2 faults (which are unlikely to happen together) before the appliance can Pikachu the user. I.e. 2 layers of protection

The first layer is always the appliance's primary isolation - First layer of isolation on wires, Insulation sheets around coils (in motors or ballasts), sand fill in heating elements, and so on

The second layer is a backup if the first fails. It can be Earthing (to catch the fault and provide a safe current path for limited current faults, or trip breakers for unlimited current faults), or second isolation layer that still contains the fault

The second isolation may be present in metal body appliances - If there are 2 layers of isolation everywhere between the live parts and enclosure

Old appliances did not have such requirement - In many of them (that were not considered particularly prone to primary isolation failures), the primary isolation alone was all thats standing between live parts and the user

It seems about right for that change to have happened around the 70s

You've answered your own question, really. If an item is double insulated, then it doesn't need earthing/grounding as there is no possibility of live conductors becoming accessible, at least not without deliberate intervention like you taking it apart while it's still plugged in