At least part of the reason (I’m saying this without having looked at the conversations surrounding the formulation of c. 917) was that the allowance for a second reception centered on those times when the faithful would attend more than one Mass on one day, e.g., a Saturday morning Mass and then the Mass of obligation for Sunday, two Easter Sunday or Christmas Masses, Chrism Mass and Mass of the Lord’s Supper, etc. The original relaxation from the old law (one reception per day, except in a couple extraordinary situations), then, was for another reception at another Mass. There was never an allowance given for a second reception at anything other than Mass (except for viaticum and times when there was a danger of profanation of the Eucharist–these are the extraordinary circumstances).
The 1983 Code got rid of all the individual allowances for a second reception (see, for example, the 1973 document _Immensae caritatis_and its many scenarios) and just said a second reception is permitted as long as, in accord with the origin of the permissions, the second reception was at a Mass.
The faithful are never required to attend a “Communion service” but they are sometimes fulfilling obligations by attending a second Mass in one day. It is fitting that they receive Holy Communion at such Masses.
Dan