If you already design stereo amplifiers making a mono version is simple. Just remove one channel and its input and output connectors from the chassis, change its name, and you're done. But what if you were starting from scratch? How much different would your design be than simply eviscerating a stereo model? Much would depend on the design goals of the amplifier, and many of those goals come from expectations ingrained in consumer minds, and not always for the right reasons.
When purchasing decisions are made folks typically turn first to wattage ratings on the specification sheet. It is generally expected that a mono amplifier should have more power than a stereo model, a myth perpetrated by amplifier manufacturers. Why buy a mono amplifier with identical specs to its stereo counterpart?
Power amplifiers are generally selected to match the power requirements of the loads they need to drive, though it has long been understood it is ok to go bigger, just never smaller than needed. With this in mind the challenge faced by amplifier manufacturers of the 1960s and 1970s was to give consumers the same rated power monos had in one stereo chassis. No one expected more, they were just happy when it was the same. Once the shift from monos to stereos took place and enthusiasts and manufacturers returned to mono amplifier designs to coax more life out of the system, there had to be good reasons to make the switch, and increased power seemed an obvious distinction. It is difficult to quantify more lifelike sound on a spec sheet. Yet, the requirements of loudspeakers are no different today than when the world was heard through a single channel.
If an amplifier manufacturer started a mono amplifier design from scratch, he would choose first the wattage goal, followed by the topology, lastly the mode of operation. For purposes of this post it is easiest to explain a solid state class A/B amplifier with a conventional power supply. The components are straightforward: power supply, amplifier circuit, chassis with adequate heat sinks, input and output connectors. This list is no different than that of a stereo design, just one channel short and a smaller power supply. The advantages of building the mono amplifier from scratch are cleanliness of design, and a power supply that matches the design goals set forth.
But most amplifier manufacturers don't start with a clean sheet when designing a mono amplifier. Today's designers almost always modify a stereo amp instead, hopefully using as many common parts as possible (so the company's purchasing agent doesn't have a fit). Which is why mono amplifiers usually have more power than their stereo forefathers; it is easier to use a stereo sized power supply to build the mono version.