As a resident of the USA, I have noticed that it is quite common to see that older street lights were often used for illuminating parking lots such as GE M400, GE M1000, Westinghouse OV50, and Westinghouse OV20 cobrahead street lights. Additionally, I have also seen some Canadian members who have posted photos of street lights being used for parking lots lighting as well. Why was it so common for North American parking lots to use street lights as their source of illumination?

Well, what else would you use? A yard light?
On the lots I have seen that use things other than street lights, it looks really weird.

I would say the "replacement" for street lights in parking lots are shoeboxes.

I would think that most of these plazas w/ street lights were built in the 50s / early 60s before shoeboxes became popular products.

There are heaps of government owned and maintained car parks here in Canberra, they all have regularly maintained 10m metal columns and twin 4.5m brackets with 110w streetlights fitted. From the 70s to 2009 there were 8.5m columns with either 400w mbf/son post tops or 2 or 3 400w mbf lamp post tops. The latter I'm only speculating the lamp configurations because I've only actually seen one lit, and that sure didn't tell me anything! They are just larger versions of the single lamp fixtures. I can post a streetview link if anyone wants.

I would say the "replacement" for street lights in parking lots are shoeboxes.

I would think that most of these plazas w/ street lights were built in the 50s / early 60s before shoeboxes became popular products.

That is exactly correct. Shoeboxes didn't become a thing until the early 1970's. Even back then, they weren't extremely popular due to how many existing installations of street lights in parking lots were.

The only two options before shoeboxes came out were either cobraheads/clamshells or those gigantic 4,000W post-top area lighters. However, those weren't nearly as popular due to how expensive they were.

When shoeboxes started to become popular in the 1980's, "pole-arm-and-light" setups began to be phased out. They were much better in nearly every aspect: cost, serviceability, less maintenance, and optical efficiency.
 
However there are still some holdouts of "pole-arm-and-light" installations that were never taken down and merely got replacement lights as time went on.

Well, I've seen almost every type of outdoor light fixture used in parking lots over the years. From residential grade incandescent to full blown 1kW MH floods, through decorative Holophane and the like, ballast-in-arm, all types of cobra heads, shoeboxes, etc. in all technologies (well maybe not carbon arc, but... )

I believe most parking lot decision makers don't really care what the outcome looks like, just so long as it works. They ask contractors to supply suitable lighting, and the contractor works out the lowest cost. Sometimes they reuse existing wood poles (especially private lots) and they find suitable fixtures to install. Electrical counters can get any fixture you want, be it shoe boxes or streetlights.

When I bought my Cooper Vanguard, it was a regular stock item. They had to "special order" it from one of their other locations because the mercury vapour ones were getting sold out. If I wanted HPS, it would have been in stock right there.

It was probably a cheap way to light a large area, and was probably cheaper than more decorative solutions. Interestingly the older shoeboxes were built fairly similar to cobraheads, usually even sharing the same optics but in a rectangular housing instead of a teardrop shaped one.

I suppose the Styled Mercury series from Line Material was a early attempt to market more decorative luminaires for use in parking lots but just like the early shoeboxes they shared a lot of optics with cobraheads.

It also seems like most major manufacturer had some form of the 4x1000w lights, Wide-Lite, Sylvania, GE, Powerlite, and Holophane all had some form of 4x1000w light or hardware to mount standard 1000w lights in such a manner.

The pole and arm style parking lot lights seems to have originated with incandescent gumballs, later moving to fluorescetns, MV clamshells, MV cobraheads. Later on some of those parking lots got HPS cobraheads or MH cobraheads. Some even got LED fixtures, sometimes generic LEDs, but sometimes utility grade stuff such as NXTs.

I've also seen some large parking lots use highmast lighting for lighting, although that practice seems to be fairly rare.

Some government owned and built parking lots, such as those found at transit stations, public parking lots, and carpool lots were built fairly recently and still use the pole and arm style lighting. But with newer HPS and LED luminaires. I suppose it makes sense since they could piggy back on the luminaires used for roadway lighting instead of ordering a set of shoeboxes for the parking loot.

In my area, street lights in parking lots are rather uncommon except for government owned places (like school)-