As of April 21st, this help article was updated, which explains how you can be notified when the expansion is able to be purchased on the Playstation Store.
I don't have an exact reason for why it's not available, but if I had to guess, Sony typically takes a while to process store requests, whether it be for game updates or store additions. Zenimax might be having issues getting it finalized, which is why they opted for the 'Notify me' fix in the meantime.
Edited by MilkJugg24 on 28 April 2020 21:50
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