It was just a holdover from the constant need/desire to have 15 of 21 sign. If we wanted to use a dynamic signing threshold - which is what using eosio.prods gives us - we could also assign authority to two other child permissions of .prods. There is .major which needs of 11 of 21, and there is also .minor which needs 8 of 21.
That is an interesting thing you bring up. The hope though is that by the time that Referendum's are being really utilized by the community that it is NOT the BPs who have to run this, but trusted community members/community groups.