@Mr_PT Cause if it's one on one the author has to actually invest into character development .
In most harem you can just write one dimensional girls like the pervert one,the tsundere one , the shy one, the one that doesn't like mc until he does some white knight shit and so forth..
From there just endless teases towards MC that wound up to nothing cause the moment he is presented with too many choices he acts like all harem mcs do he rejects making a choice and instead keeps all the girls chasing him till the end of the story without outwards rejecting a single one.