Go back to the start and middle of the '70s. It started with one of the first two serious big honkin' bass amps. When Ampeg first made the SVT bass cabinet, they went for mulitple small drivers to provide projection, move a lot of air somewhat more efficiently, and to tailor the frequency response. The other big player at that time was Acoustic Control who went with a single 15" in a folded horn. That threw the bass further, but on stage it was hard to hear. It took a good distance for the waves from the two sides of the W box in a folded horn to focus and be heard.
But even after those two were popular, a lot of bar-band bass rigs were still based on 15 inch speakers based on the (erroneous) assumption that you need bigger speakers for bass. Then Larry Hartke made a bunch of 10" aluminium speakers and stuffed 'em into an SVT cabinet for some has-been nut case named Jaco. When Jaco hooked up with Guild to endorse the Pilot bass, he talked to Guild about the Hartke speakers. Guild and Hartke got together and made the first popular and good sounding 4x10 cabinet because they thought an 8x10 would be limiting the potential market. Between the sound of the 4x10 configuration and the very visible look of the Guild/Hartke, 4x10's got to be really hip. So lots of others started making 4x10s. David Nordschow (The Founder of Eden) was marketing PA gear when he kept hearing about the Hartkes. He figured there was a better way than what Hartke did, so he designed a speaker and cabinet which became SWR's original Goliath. From there the 4x10 configuration took off like crazy.
You can put four 10" speakers into a box that's the same size as a single 15, get a different sound (in my opinion it's a much better sound). Multiple 10" speakers tend to have a punchier sound, without the problem of flabby sounding lows. They tend to sound tighter and more focused, especially on stage.
Guitarists like 12" speakers because that's the typical guitar amp sound. Until SRV and all his miserable clones got popular, the most common amps guitarist aspired to were a Fender Twin Reverb (a pair of 12's), a Fende Deluxe Reverb (a single 12- a VERY popular studio amp), and the Marshall-style 4x12 boxes. BTW, the Marshall 4x12 started as an 8x12 for Pete Townsend. His roadies threatened to kill him because the cabinets were so big, so he had Jim Marshall cut them down in half.
While there were guitarist using Super Reverbs, and old tweed Bassman combos (both were 4x10 configurations) as well as Princetons (1x10), it wasn't until SRV did for blues what Kenny G did for jazz that we started seeing lots of folks wanting 10"s for guitar.
So, like most things in this screwy business, it starts with someone (designer and/or musician) coming up with something that sounds good to them, then it moves into being a fashion thing when lots of people start emulating their heros, then it moves into either remaining popular (4x10, 1x15, etc.) or falling by the wayside (folded horns, brass hardware) depending on how it actually works.
jte
