Well, lets see...
There are 4 ways you can make a wrench, from cheap to expensive..
1) Stamp it from sheet metal
2) Cast it
3) Drop forge it
4) Machine it from a solid block of metal
5) Make it blacksmith style
But, in the mid-late 1800's..there are not yet any high speed stamping machines, drop forging is fairly new, and still labor intensive, *and* it only works with steel. Steel is expensive.
Making it blacksmith style, hammering, shaping cutting , filing, you are going to take days to make a few dozen wrenches. Casting them, you can make 30 dozen in one pour, and a decent foundry will do 20 pours a day...and they are cast iron, much cheaper than steel...
So, yes, cheapness.
But also design and advertising, an odd side effect of casting them is you can easily put your name on them, and decorate them, and it doesn't cost anything extra...
I suspect that, for some companies, this was probably a deciding factor even after they knew they should probably be providing a better quality of wrench..