Why do Preppers Buy So Many ARs?

Preppers buy truck loads of cheap AR15s. You'll see Preppers with tons of these guns (And, AKs, too), especially in the US. They stock pile the ammo along with the guns. That's on top of everything else they stock up, including food and water. The question is, for many people, why? Here's my best attempt to answer that:

1. Low Cost - Preppers are going to stockpile SOMETHING for self defense, so they look for things they can stockpile cheaply and effectively. Enter the AR15. In the US, they are extremely plentiful and they are very affordable for relatively high quality rifles/carbines. So is the military surplus ammo that they use. In terms of cheaply purchasing quality rifles for self defense and hunting, you won't beat the AR15 in the US. Compare to the Tavor or MCX for cost and the cheap Yugo AKs or old/cheap FALs for quality.
2. Spare Parts - You also have to consider the availability and cost of spare parts for repairs. AR15s are so plentiful and available, that you can go to any major city in the US and find an AR and their parts for sale. That kind of availability provides you with something Preppers crave: large sources of needed materials and products. Bolts carrier groups for $100 or less, complete lower parts kits for $30, spring kits for just a few bucks - cheap and available replacement parts. And, they're good quality, too.
3. Friends and Family - Not every AR is an daily shooter. Some of these are being bought so that a friend or family member will actually have a rifle to help hold down the fort after they appear, looking for food and shelter. If you prefer an AR, then having a gun that shoots the same ammo and uses the same mags for these friends or family is a huge plus.
4. Ease of Use - There is a general consensus that the AR is extremely easy to use - so easy a child can use it... and they do. There are a number of weapon options that fit that bill, but none so familiar to the US populace or as ergonomic. Plus, ARs tend to have easier recoil and  lighter weight than their alternatives, for weaker shooters. (Compare to the AK.)
5. Ammo Cost and Availability - There are only a few rifle calibers that are this cheap and even fewer that are this widely available in the US. In fact, .223/5.56 is the most plentiful ammo for a defensive rifle in the US. It's tough to beat .22lr for cost, but this is the standard for self defense in this country and this continent.
6. Compatibility with the Military - If we are going to form a militia or work with ex military units to help fight some invading force or help to restore rule of law, being able to use military ammo reserves and share magazines is hugely beneficial. Exotic calibers are going to be hard to get resupplied.
7. Availability via Scavenging - Another benefit of these being so popular and being used by the military is that the dead will leave behind ammo and compatible parts to use in or repair any ARs you own. It bears specific mention because you never know how bad you may need to find something if SHTF or WROL ever happens.
8. Reliability - Despite what some crazy guys who still latch onto the early problems of the AR in Vietnam will tell you, the AR is reliable and durable. Filthy 14 went 40,000 rounds without much problem or cleaning and my experience has led me to believe that with these better-than-space-age materials being used today actually make the AR platform ridiculously reliable. Don't think an AR is a jam waiting to happen - they will run for a LONG time with very little upkeep! Besides, who has 40k worth of stored bullets and will shoot them through a single gun? (Really, how many people out there will honestly shoot 40k rounds of anything in their lifetime? You'd be replacing the barrel long before that point, typically.) This is a durable machine we're talking about - even the cheap ones!

Are there other guns that would serve you well in a WROL or SHTF? Sure, but the AR, for better or worse, is the king of these rifles for prepping in the US because nothing scores as well, in total, when considering all the above points. If you favor another platform, stick with what works for you, but that is why Preppers are buying so many cheap ARs. Which country you live in and the applicable laws in your area have a huge effect on what will work best for you. I recommend you weigh the options and get something - because something is better than nothing in this case.