Cats respond to a name on their own schedule, and a short one at least gets said twice in the time a long one takes once. Four letters or fewer is the definition here and it is computed from the spelling, so the filter is exact rather than editorial.
Why so many end in a vowel
Because that is what people choose. It is not a rule imposed on this list — it is what the registers contain, and it holds across every city counted.
Every count on this site is a tally of real registrations: pet licences and shelter intakes published as open data by New York, Seattle, Austin, Montgomery County, Cambridge, Zurich, Basel and three Australian councils. Popularity here is the average share a name holds across those registers, so a name common in a small city is not buried by one common in a large one. Nine of the eleven sources are North American, so read the ordering as naming in those places rather than worldwide.