It's a somewhat common sentiment when talking about space exploration or deep ocean exploration:
Fewer people have reached the deepest point in the world's oceans than
have walked on the moon,
This is comparing apples and oranges.
- a single small valley that happens to be the deepest part of the ocean, vs.
- the entire moon with all its variations in geology.
The surface area alone suggests more science can be done on the Moon than in the Mariana Trench.
Fairer comparisons would be the Mariana Trench vs. Pluto, or the Moon vs. the entire ocean floor area deeper than 2 km.
For a small area, a few visits will be enough to get a good science return. When you survey a large area, you need more visits to get a good idea of the variations in that area.
Also, we've done a lot of geology and biology surveys on Earth already and we're pretty good at predicting what a place will look like based on other, similar places. We've been to the ocean floor many times, just not that often to that particular place.
The Moon, however, is very different from Earth and presents an environment we've never seen before. That warrants at least a few visits to get data from different regions.