Because you can hold a candidate to a precise statement. You can't hold a candidate to an evasion or a circumlocution or to a vague generality or a clever idiom. You can't evaluate how well a candidate "Makes America Great Again" but you can evaluate whether they reduced the number of shootings by cops per capita or if they reduced wealth inequality.
Whenever the 3 Dem candidates are on stage together in a debate, it feels like a united front of one party. Whereas during the republican debates, it's like 10 people constantly trying to step over each other for the limelight.