Some answers to a roughly similiar question here: https://www.reddit.com/r/AskHistorians/comments/8npilo/the_roman_senate_continued_to_meet_more_than_200/

but more specifically after the deposition of Romulus Augustus (last sovereign Western Roman Emperor), Rome was in the hands of Odoacer, whose power base was to the East and North i.e. away from the Roman Senate. It was much more common for these rulers to seek alliances among other power-holders in the region, including non-Romans, so the Senate was seen as the formal placeholder of power. It continued to meet for 200 years or more

EDIT: was not aware of rules. u/bitparity for earlier comment

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