If I'm to be honest most of them went to shit when TS4 came out. One of the reasons is that this game....just isn't good. It's entertaining for one or two hours then it hits you with boredom from it's repetitive mechanics, handholding, poorly designed gameplay features, lack of gameplay, limited worlds and lots and so on. The only good things it has are build mode and Cas and it's what most youtubers are doing now....no one can hold a let's play now, unlike before. It's true that the attention span of many people shifted towards more quick "dramatized" videos and youtubers follow what gives them the most ad revenue but the game itself doesn't help.
TS2 and TS3 let's plays are still to this day more prominent, because the gameplay is interesting and after a while we all get bored of watching some random people build things in TS4.