Mostly the performance characteristics. A virtual machine presenting as a machine needs an operating system to be useful. Most operating systems have long-engrained assumptions about the nature of the world, such as:

"There is a time when I go from power-off to power-on, and it is rare, so I may perform expensive operations then to amortise their cost over running time".

or

"While running, time does not skip and hardware does not change".

The practical upshot being that the OS needs to be booted from scratch in a number of scenarios.

But it's not the OS that provides value. It's a means to an end, and that end is to run software. Most software written to run on OSes also have engrained assumptions, such as "I will come to be launched on a fully-booted system".

Containers move the virtualisation up from hardware to the OS API surface. Because the cost of booting is now amortised over all containers running on the system, the original assumptions of both OS designers and software designers become, approximately, true again.

So you're right, we came full circle, but not to a point that means "use fully-dressed VMs again".