I'd guess it's just because of cost... 60/40 is close enough to eutectic, and saves a little bit of money. Wikipedia (
http://en.wikipedia.org/wiki/Solder ) says: "Sn60Pb40 is slightly cheaper and is often used instead for cost reasons, as the melting point difference is insignificant in practice."
I personally like the silver bearing solder - Sn62Pb36Ag2... it has a slightly lower melting point than eutectic, but overall just feels nice (can't really put my finger on exactly what, but I like it). I also use a really old roll of Alpha solder with no problems... from 1986. Solders better than new Radio Shack silver bearing solder (though theirs may be from the '80s as well

).
So it
is a price thing.
I've never tried Sn62Pb36Ag2 before, but I've never had any problems with the solder I used at the factory (and now use at home). It is predictable, consistent, flows and solidifies perfectly, doesn't make a mess, works perfectly with a 700°F Metcal tip (STTC-1xx), and apparently has no expiration date.
I can't remember if the normal size we used at the factory was .025" or .020" (it was definitely one of those 2; I sent an email to my friend that still works there asking about it); whatever it was, it was the perfect size for typical through-hole PCB soldering:
0.00018 sq. in. (area of 1 strand of .015" solder)
0.00035 sq. in. (area of 2 strands of .015" solder)
0.00031 sq. in. (area of 1 strand of .020" solder)
0.00049 sq. in. (area of 1 strand of .025" solder)
0.00080 sq. in. (area of 1 strand of .032" solder)
The common .032" size is way too big; that is equal to about 4.5 strands of .015" twisted together. Twisting 4 strands of .015" together is what I do for soldering flyback transformers to the board, which is a significantly heavier job than average.
On the other hand, .015" is too small for typical through-hole stuff, though it is fine for small capacitors and ICs. For more typical sized through-hole stuff I twist 2 strands together, and that is about right, and doing that is closest in size to .020", so I'm thinking .020" is what we probably used at the factory for typical through-hole (like
common terminal blocks), rather than .025".
I don't see how anyone could do e.g. a capacitor kit on a typical monitor chassis with .032" solder. That stuff is so thick relative to the size of a typical monitor chassis capacitor leg and pad, that the instant you touched it to the joint it would melt enough (or more than enough) solder into it, and then you'd have to hold your iron there until it flowed down and into the joint properly; but you wouldn't be feeding fresh solder and flux while that was happening, so it would be boiling away the existing flux. That would make it tricky to get perfect joints. A proper solder joint is made by using a solder wire size that requires you to feed the wire about an inch in order to get enough in there, because this keeps fresh flux coming steadily as the heat draws the solder down and into the joint, and allows you to remove the iron and solder wire at the same time.