On reason is that die cast (alloy wheels) generally have an odd number of spokes. This is because having directly opposed spokes causes problems with residual stress distribution as the casting cools and shrinks (this is also why cast iron hand wheels often have S shaped spokes). So 5 studs tends to be a more convenient number, if only for aesthetic reasons.
Wheels with an even number of spokes do exist but these are generally machined from a solid billet in several parts and are substantially more expensive than die cast ones.
Nowadays most vehicles are designed on a common platform which is used across a wide range and even things like vans often have alloys wheels as an option so there is little point having different hub designs for steel and alloy wheels.
There is also the issue of redundancy. If one of five or six studs fails or is loose then you still have a reasonable distribution of functional nuts ie covering more than 180 degrees of the hub. If you only have 3 out of 4 present then it is entirely possible for the wheel to not sit flat on the hub.