Welcome to the forum Ulysses. 
From https://ubuntu-mate.org/blog/ubuntu-mate-bionic-beta1/
Raspberry Pi images
We’re am planning on releasing Ubuntu MATE images for the Raspberry Pi around the time 18.04.1 is released, which should be sometime in July. It takes about a month to get the Raspberry Pi images built and tested and we simply don’t have time to do this in time for the April release of 18.04.