Why is it "sequacity" and not "sequenicity"?

sequacity and sequacious, meaning obsequiousness/servile following of others/thoughtless, uncritical acceptance is somewhat archaic.

sequenicity is ... to put it bluntly 'not a word' (meaning it is not really accepted, not in any dictionary, prescriptive, descriptive or otherwise) and is actually not used by anybody. It looks like a neologism or a malformed attempt at making a regular extension of sequence.

If the latter, then the appropriate extension of sequence to an adjective would be

sequential

and from that to a noun for the concept would be

sequentiality

Because this started from sequence, all the derivations use 'e' instead of 'a'.

Sequacity does not proceed from sequence directly. It comes from Sequacious. Both have roots in the same Latin word.

Sequacious: following with smooth or logical regularity.

Therefore, a sequence is sequacious.