I think a 12inch macbook could sell if it was powerful enough and had the right number of ports. There are many professions out there where small portable but yet powerful laptops are needed or to the point wanted. The problems have always been the same, the screen is not bright/clear/sharp/detailed enough, the battery life is poor, there is never enough ports for external devices.
One of the biggest problems for the professions that benifit from using small laptops is that many of the professions require their staff to travel a lot around a country or the world which is a problem because airline companies keep clamping down on how much hand luggage a passenger can carry and thus a small yet powerful laptop would suit them perfectly.
A 12inch macbook built with the right specs could be popular with students, small, lightweight, good lcd, good battery life, good selection of ports for external devices, something that could easily fit into their bag or backpack.
The problem for Apple is if they did build such a device the result could be that their other macbooks do not sell and Apple is not about to build a product that will affect the sales of it's other macbooks.
I’ve always used my 12-inch, both PowerBooks and MacBook Retinas, with several external hdds for video editing, with the FW400 on PowerBooks, and with USB3 on MacBooks.
Ports are a problem for those who want to have a problem: it's full of adapters around, it was full even 30 years ago with other Macs, if you want to work you work, otherwise they look for excuses.
Just like the size of the screen: those who think they are working seriously on a 15”/18” make fun of themselves, to work properly without ruining the view you need at least a 23” and, since there is no portable Mac with a similar diagonal, whether it is an 11” or an 18” you still have to connect it to an external screen.
Finally, the power/thickness/doors issue: the problem with all devices is heat dissipation. Both the 12-inch PowerBook and the 12-inch MacBook, but so do iPhones, iPads, Apple Watches, become hot under stress. If you want better dissipation you have to make them bulky and possibly with a fixed connection to the electric current, but even then under stress they could become hot.
I've been an Apple customer for decades, I still haven't found one of its products that, under stress, isn't hot or if equipped with fans doesn't become as noisy as a helicopter.
The only ones to save the Mac Pros inserted in special isolated and thermo-controlled rooms (maybe with liquid-cooled PowerMacs it was better, but I never had the pleasure of using them).
In short, they are all compromised, it's physical. Apple doesn’t even do miracles.