The problem with Bristol airport is it has number of low cost and holiday airlines doing two out and back flights each day, all compressed into few manic hours
Loads of flights go out 6am-8am, upto 4 flights timetabled for same 5 minute departure time (and yes there is only one runway), then only a handful of flights 9am -1pm, then another manic batch early afternoon as the early flights return (so congestion in arrivals, and busy departures), once these have departed, is quiet until another big block of arrivals in the evening.
Per CAA airport figures
Bristol in August 1,217,472 passengers,
Cardiff in August 123,785 passengers
so Bristol nearly 10 times as busy.
The other problem with Bristol is often means check in is around 4am or 5am, or return is nearer 10pm - 11pm and apart from buses to centre of Bristol, nigh on impossible to get a train to connect for these times.
Most recent CAA published month (November) is
Bristol 616,807 passengers, Cardiff 47,278
which shows how how much variation due to seasonal demand.
Clearly can't justify a Metro to Cardiff airport when during quiet months only 1600 passengers per day.