I'm not a Dev so I can only make a guess but I think it's related to the fact that after finishing an STF, you're on a 60 minute cooldown and those usually take about 15 minutes to finish...if all goes well (and you don't outright fail).
Fleet Actions like SB24 take about 5 minutes, you can't really fail and they're on a 30 minute cooldown.
considering most people are 50 i dont think it matters that much.
but the dilithium added to fleet actions was in place of stf's which was endgame content. lower rank players did benefit greatly from this and they still benefit to a degree but the bulk of the playerbase are now max rank. that is where they will focus their attention.