A borrowed scale
The ten-point must system awards the round winner ten points and the loser nine or fewer, a scale built for boxing where damage and clean punches are the whole picture. Mixed martial arts asks the same scale to account for takedowns, control time, submission attempts and strikes from multiple positions, which is a much wider set of things to weigh against one another. The scale did not change to fit the sport; the sport was fitted to the scale.
Round isolation versus fight momentum
Because each round is scored on its own, a fighter can dominate four minutes of a round, get taken down and controlled for the last minute, and lose that round outright under most interpretations of effective grappling. Judges are not permitted to average across rounds or credit a fighter for the shape of the whole fight, only for what happened inside that five-minute window. This produces split decisions that feel wrong to viewers who were tracking the fight as a whole rather than round by round.
Where the criticism lands
The hardest rounds to score are close ones with mixed output: a fighter who strikes more accurately against one who controls more position and time. Neither dominance is easy to convert into points, and different judges weight them differently in good faith. Proposed fixes such as half-point scoring or separate grappling criteria address parts of this, but none has replaced the ten-point must system, largely because its simplicity is also its main appeal to commissions and broadcasters.