Economics/Practicality
When I started my gun collection I did actually do so with a logical plan (wish I could say so across the board in my life but I digress
). Don't get me wrong, if I had Donald Trump like money I would buy whatever struck my fancy but that doesn't work for me in the real world.
Thus I focused on three primary calibers for the past thirty years. 9mm because it is perhaps the most common easily had pistol caliber in the world while simultaneously being cheap relatively speaking. .22lr because it too is one of the most easily found and affordable calibers in this country at least. What the 9mm and .22lr have in common as well it that I have each caliber in both hand and long guns meaning their quite practical. Lastly but not least in any sense is what I see as OBLIGATORY for anyone wishing to have a survival or 'what if' gun, namely, the ever popular for good reason 12G Shotgun--It allows one to either use for CQB, HD/SD bird-shot, slug, buck, wax, non-lethal etc. ammo diversity. Also, I think many people forget that a skilled shotgun marksman can actually use it to sniper like effectiveness up to 150yds if need be (seriously).
To sum it up for me, I choose the aforementioned three calibers because they are both plentiful in supply, affordable and extremely practical. Oh, and I have a bit of .45acp as who could not have a 1911 in their collection modest as mine may be compared to some of you guys (I have a carbine as well).
I know what some of you are thinking, how can this guy not have .223 , 7.62 etc...
? I have just always used either my 12g or .22lr rifles for long distance and I have 9 and 45 carbines for CQB stuff. If I had more mula I would have an AR and AK for sure though...
YMMV.
-Cheers