In general,  the low earners have a low volume of sales rather than just a few niches they serve. There are specialty agencies in the macro/RM world, but by and large the microstock agencies are general collections.

The top tier are better at it than the others - higher sales volume.

I can give you a very very old example of my own when an image was featured on BigStock's front page slide show (before Tim & Dawn sold to Shutterstock) over the Christmas period and my November income that year was larger from BigStock than Shutterstock. SS wasn't as dominant then either, which helped. But that one-time fluke meant nothing overall or over time.

If you can find a popular style of illustration - start some new trend - you could do very well for a bit until the copies appear, but I don't think there's much hope to be pinned on the low earners to make that happen