[DanEC]

Most likely because that is where auto manufacturer's wanted a port for connection to retard low speed timing to increase engine heat and decrease emissions at the expense of performance, fuel economy and lower cooling temperatures. I doubt any of that (poor low speed performance, high cooling temps, poor fuel economy) appeals to you which is why everyone interested in a performance engine connects their vacuum advance to a full time port originating below the throttle blades. Since back in the 60s Holley typically had one port above the throttle blades and one (ported) below and since the full time vacuum port was often required for the choke pull off vac can, it became common to cut that hose to the choke pull off, put a "T" in it and connect it to the distributor vac can also. Then cap the useless ported outlet.