In 1916 a miner's hourly wage (to pick one I could find online quickly) was about $0.19.5/hr. or about $7.80 a week assuming a 40 hr week (and I'd bet they worked more like 50 - 60 hrs./wk back then).
In an old 1908 Sears, Roebuck (repro) catalog I've got on the bookshelf, a brand new Colt Cowboy Frontier revolver (SAA or Bisley) cost $15.50 (big sigh), as did a Winchester 1894 rifle (.30-30, w/ 26" octagonal barrel... sigh) but other rifles, like a Trap Door Springfield Gov't .45-70, ran about $2.75 (and they threw in 20 rds of ammo to boot) (and yes it was a surplus rifle bought from Uncle Sam's stocks).
So lets say today you earn minimum wage or round up to $6.00/hr. (easier math for me) $6 x 40 = $240.00/week. Let's forget about buying a new Colt SAA for a minute tho' since it kinda skews things and since Winchester no longer makes, uh, well, let's forget about them and substitute a S&W Model 10 and a Marlin 336. Davidson's listing for each: $499.00 for the Smith or a Marlin 336 at $359.00. Maybe we should use a Moison Nagent or a Swiss K-31 to compare w/ the nasty old surplus Trap Door... (another sigh) at $100.00.
Hmmmm. Looks like about 2+ weeks of your life is tied up in that revolver whether you bought something back then, or today and a little less time to buy a good enough rifle (OK, the Marlin qualifies as a great rifle).
But, to get to the heart of the matter, and as others have pointed out, the labor costs associated with manufacturing one handgun is typically greater than the labor costs associated with the longer arm. Material costs are really minimal in the overall scheme of things but they aren't getting any cheaper. Factor in shipping from overseas. Factor in Supply & Demand. Also, factor in Overhead & Profit for each entity along the way. And last but not least, factor in taxation at each step along the way. (and gov't surplus is a great way to go if you really want something cheap)
A final point. Oversea labor is usually a lot cheaper than US labor with some exceptions, notably European and Japan.
Other than that, I can't think of a really good reason.