A new platform on which to play games is invented. It might be a new graphics technology (Vector graphics! Color! 3d! VR!). It might be a technical advancement of a different sort (Modems! Servers! Streaming! D-pads! Small screens! Big screens! Touch screens!). It might just be a new marketing channel (Games in bars! Games at home! Games in restaurants! Games in stadiums!).
Its distinguishing characteristic is that it is worse at the old sorts of games than the existing platforms, but better at something new.
It’s still cheap to make something for it, usually, and it’s risky. Big companies stay away, or they try porting over something that has worked before. It doesn’t do great because it’s a mismatch for the new capabilities — and restrictions — of the new platform.
Small companies make something that fits the new platform well. Maybe it has the right controls, because the new platform offers something new. Maybe it has the right interface, or the right play session length, because of the new platform demands on the player.
It’s almost inevitably something new in mechanics, with fresh game system design in some fashion. It has to be, you see, to take advantage of what the new platform offers.