I guess we'll ignore the tens of thousands of Fender combos that have horizontal chassis with the controls in front and the power cord and output cable at the back. With solid state amps, there's no particular reason to put the controls at the back except for form factor, IMO. It may be slightly cheaper to fabricate the case, but I suspect the reasons are mostly aesthetic. Retro look, everyone else is doing it, etc. One of the reasons I prefer my old first-generation Fender Rumble practice amps is the controls are on the front and I can park stuff on top of them without it being in the dang way. Plus there's a lot less potential of an adult beverage being spilled into the controls.