From Zom's build metadata file (https://gitlab.com/fdroid/fdroiddata/blob/master/metadata/im.zom.messenger.txt): the latest version requires proprietary dependencies which isn't allowed in F-Droid. Additionally, the goal is to include the original .apk signed by Zom's developers, but so far we weren't able to reproduce their .apk.
Help on bringing Zom into F-Droid is highly appreciated!
CC: @thoughtcrime @switchingsocial @mattlaff @skynebula @ajroach42