Hi all I am only occasional user of Debian however I believe that when I've installed Debian 9 with espeakup enabled the installed DI was Mate. Today I've installed Buster RC1 using iso downloaded from https://caesar.ftp.acc.umu.se/cdimage/buster_di_rc1/amd64/iso-cd/debian-buster-DI-rc1-amd64-netinst.iso and Gnome was installed as a default DI. If the change was intentional simply ignore my message but I believe that it is a bug and the intention is to install Mate by default.
-- Greetings Lukasz