while it's true that record companies can't "put" records into stores, they can certainly provide the promotional material to draw attention to releases and artists. The missing info none of us have is the annual promotional and marketing budgets allocated to the supremes as recording artists, year by year.

trade magazine ads
pre-recorded radio spots
touring and visits to top market dj and radio programmers for on-air interviews
in-store display stands
posters, stickers, other instore materials
press conferences
promotional appearances at industry trade shows/conventions
outdoor media space
etc

like any corporation, motown had an annual budget that was divided between departments, one being promotion and marketing. then this dept would take that and divide it among groups and/or by month. as releases were being prepared, these teams would do their work at preparing campaigns to support releases. if an artist wasn't forecast to generate a huge amount of sales, then rightfully their promotional budget wouldn't be as much.

since we don't have this info, we have to simply go on what we can gather. in 70 and some of 71, it appears that the volume being spent on the girls was similar to DRATS years or, at the least, DFM years. as public perception cooled, money appears to have been decreased, i'm guessing.