Using d-i command to install packages for unatttended installation
If you are using the Ubiquity installer (the desktop image) then the wiki states that pkgsel and tasksel preseed settings will be ignored.
From https://wiki.ubuntu.com/UbiquityAutomation
Preseeding keys for the following installer components will not be used in Ubiquity, usually because they do not fit with Ubiquity's mode of operation:
netcfg
LVM and RAID partitioning
base-installer
pkgsel/tasksel
finish-install