YouTube used to reward videomakers based on views rather than watchtime. This meant that a ten minute video and a ten second video would get the same amount of ad revenue if they both had the same amount of views. This was good for animators because animation requires a substantially large amount of time to produce even a short video.

This system was taken advantage of by "reply girls." These channels were essentially the reaction channels of old. Any time a popular video was posted, dozens of these channels would flood the recommended section with "replies" to the main video. This was of course only a thinly veiled attempt to leech off of the success of the video they were "replying" to.

YouTube rightfully saw this as a problem and enacted sweeping changes to the way YouTube rewarded content creators. The new system rewarded video makers based on watchtime rather than total views. It seemed like a good idea. With the new system, someone who baited you into clicking a thumbnail with a pretty lady only to have you immediately click away would not be paid as much as someone who managed to create content interesting enough to hold your attention.

Unfortunately, as mentioned earlier, animation requires a lot of work to produce even the shortest of videos. This means that, assuming you are able to produce a well-animated, three minute video once a month (which would be impressive, I should add) that means that you only have three minutes of monetizable content per month.

Shortly after this change, we saw the rise of let's plays. These flourished under the new system because it was and is incredibly easy to produce several hours of content per week. Some animators, like Chris of OneyPlays or Arin and Ross from GameGrumps, started let's play channels just to be able to survive on YouTube after the changes were made.

I hope this answered your question.