Yeah, so this is one of the reasons I've ended up with my amp preferences that I do have. Over the years I was writing stuff where I needed a super clean tone, a slightly crunchy clean tone, a low-gain rhythm, and a high-gain rhythm.
I started off with the Laney VH100R. A two channel amp, but each channel has a built in boost. So you can quite easily get the 4 combinations above. I used the amp for a number of years and it sorta dialed in my preferences, and started my journey there.
Then I got a Fryette Sig:X because the Laney was a bit noisy, and it offered me 6 different sounds - 3 channels, but each with a footswitchable boost. The Fryette was quieter than the Laney too. But totally different sound.
Then I got a Marshall JVM410HJS - the Satriani one. They removed the crappy built in reverb from the regular JVM, changed the clean channel so it matched the one from the Marshall 6100 amp, made the two gain channels exactly the same, lowered the footswitch latency, and added built in noise-gates. WHAT. AN. AMP. It's frankly amazing how much stuff they've put into it. The footswitch it comes with has 6 switches on it, and you can program any of the 12 modes on the amp directly to the switches in preset mode. So it's basically like a proper 6 channel valve amp. Comes at a cost mind you, because you positively have to remember to set every single knob on the amp to where it needs to be!
I have a Diezel VH4 as well. 4 channel amp with midi. The Diezel D-Moll is a 3 channel amp with midi. The Diezel Hagen, again a 4 channel amp with midi.
I have an Orange Rockerverb MKIII too. It only has two channels, but they're quite flexible. With some clever usage of boost and OD pedals, you can get the same results as the above... but it is more of a ballache.
The reason - I think - that a lot of amps end up being 2 channel amps, with perhaps some modes, but effectively the topology is just 2 channels... is because that is the easiest thing to build. When you start thinking about midi and switching modes and stuff, you then need to f**k around encorporating micro controllers and coding for them into your amp design.
I've tried a ton of amps over the years:
- JCM800 - not flexible enough, but sound great.
- JCM900 - two channels, but you can't switch the modes on them.
- DSL100 - the older model didn't allow you any clever switching stuff. Haven't tried the newer ones.
- EVH5153 - The 50-watters are great and have midi switching. So 3 channel midi switching amp right there. But version 1 of this amp has a really f*****g annoying volume jump between green and blue channels. They fixed it by giving you concentric pots on the version 2's.
Ultimately I think it is a combination of most guitarists not using that many channels, and also most amp manufacturers assuming you'll get where you want to get with the volume control and pedals.