Why is the swingarm design uncommon / rare on full suspension bicycles?
This swing arm design has fallen into near complete disuse in the bicycle world for 2 reasons:


First, there is translated suspension activation from pedaling. This design does not isolate pedaling or other rider generated forces from ground generated forces. Because of that, there is a loss of efficiency which makes it a poor choice for a bicycle suspension design.
Second, the travel path of a single pivot swing arm carries the wheel forward as it moves through its travel path, because of the arc described by the wheel. If your rear brake is in use, the wheel is prevented from following that arc, and some or all of your suspension travel is lost until the brake is released. In a 4 bar linkage design, or similar vertical travel design, brake activation does not prevent suspension activation. Thus it has become the default design for high quality dual suspension bikes.