The log scale creates that downward-sloping pattern as an artifact - it appears even if the craters are purely random (uniformly distributed across time).

Simplified example: suppose that we treat crater diameter as binary, with a 10km or more crater counting as a "big crater" and anything smaller getting ignored. If we get one "big crater" every 2 Myr, on average, then we'd expect the right half of the x-axis to be blank; the rightmost datapoint would be around the 1 Myr mark. Between 1-10 Myr we'd expect to see a few dots (4-5 big impacts). To the left of the 10 Myr mark, the dots would get denser and denser; there would be hundreds of them between 100 & 1000 Myr.

If we instead chose a smaller cutoff for what counts as a "bit crater" - say, a once every 0.2 Myr sized crater (which is perhaps a 2km diameter) - then the pattern would look the same, but shifted over to the right (by one tick mark, in that case).

In the two-dimensional log-log graph, that pattern (of increasing density to the left of the graph, petering out at different x-values depending on what size crater you're looking for) translates into the downward slope that we see here.