Natural talent, style, willingness to put in the effort, genre of work, and time allotted.
A lot of things effect work and isn't always fair to compare mangaka directly to each other.
For example just because someone is drawing 4koma or a gag-strip in a simplified style doesn't mean they are a bad artist, it just doesn't make sense for them to draw hyper-detailed art it would just clutter their strip. Likewise artists who are in Jump or some-other weekly serialized magazines may end up drawing with no background or simplify their art in an effort to meet deadlines.
Of course natural talent is a big part, but the type of manga and it's production is important to keep in mind.