To add to what you said: 50,000 visitors in 3 years is much less than 50,000 visitors in one month for example. From my experience sites with low traffic do not receive the same high quality ads as the top sites. This is done so that top publishers always have revenue and also because is harder to detect fraud on low traffic.

Also, the numer of ads to be served on the network varies and when it drops, the lower quality, lower traffic sites are the first to remain without ads that convert and so without revenue.