In order to offer generic robust rollbacks, Mender requires two rootfs partitions, which means effectively doubling the storage requirements for the rootfs.
You can see the full partition layout in the documentation.
The most important design principles of Mender are 1) robustness and 2) ease of use.
Currently, there is no other known approach that provides the generic robustness and simplicity of the dual rootfs approach.
Secondly, the price of embedded storage is rapidly declining. This is why Mender takes a dual rootfs approach first.
In the future, other and less robust update mechanisms, such as variations of packages, tarballs, will be supported as well.
Also see the question on application-level updates, as well as sensors and smaller devices below.