Breakthrough has too many players. Rush has less players and works a lot better with the squad mechanics of BFV. Squad conquest also works just as good with the squad style team play. Breakthrough feels chaotic and less organized.
And fewer players in each mode means it will be harder to find matches, which means some impatient players will just switch to Conquest where it’s always easy to find a match. Now there are even less people playing Frontlines and Rush so it will be even harder to get a match going.