Why traffic lights in the US and Canada, uses 8" or 200mm lights, instead of 12" or 300mm lights as used in the rest part of the world?

I think they use a mix of both, don’t they?  Same as in much of Europe, for instance in Belgium we have both 200mm and 300mm.  One of my first jobs for Sylvania in the mid-2000s was to develop the LED traffic signal modules, and we made those in Germany in both 200mm and 300mm for EU markets.  My sales colleagues made up a table of which diameters were used in which % for each EU country, and it is clear there is quite a mixture of both.  Many EU countries also have both diameters on a single signal consisting of 300mm for red and 200mm for amber and green.  Historically it seemed the 300mm was used for 70-100W incandescent lamps, and 200mm for 40-70W incandescent and 50W 12V halogen.

It's a mix of both although generally 8" lights are getting phased out in newer installations in favour of 12" lights. It was also very common here to only use the larger 12" lens for the red lamp and green arrows and use 8" for the amber ball and green ball, creating the 12-8-8 and other combination signals. Depending on where you live, 12-8-8 signals are also getting phased out for full 12" with the 8" only signals reserved only for special uses like transit vehicles and bicycles.

That being said 12-8-8 signals are still allowed and is probably still the most common signal head used in Ontario. Even 8-8-8 vehicle signals are still allowed under certain scenarios (speed limit 50km/h or less, secondary or auxiliary signal head). This is from Ontario's regulations and so this will differ by state and province.

Prior to the LED era we used 116-150w, typically 116w for arrows and pedestrian, 135w for balls for 12" lamps. 150w was the standard wattage, 135w was the energy saver lamp. 60w to 69w was used for 8" lamps. 69w was the standard wattage, 60w was the energy saver lamp. The 3M M131 and the McCain counterpart used a 150w PAR46 three prong lamp.

I haven't seen a new 8" installation in close to 30 years ! 8" signals are a rare find around here as everything has been 12" fir decades .

I think they use a mix of both, don’t they?  Same as in much of Europe, for instance in Belgium we have both 200mm and 300mm.  One of my first jobs for Sylvania in the mid-2000s was to develop the LED traffic signal modules, and we made those in Germany in both 200mm and 300mm for EU markets.  My sales colleagues made up a table of which diameters were used in which % for each EU country, and it is clear there is quite a mixture of both.  Many EU countries also have both diameters on a single signal consisting of 300mm for red and 200mm for amber and green.  Historically it seemed the 300mm was used for 70-100W incandescent lamps, and 200mm for 40-70W incandescent and 50W 12V halogen.

Here in Israel, we uses only 300mm signals for traffic lights, and only seldom for aux signals near the first vehicles (Not in every all junction), we uses 200mm or 100mm.
And we don't have traffic lights with mixed signals (300mm red and 200mm amber and green).