I blame the rise in popularity in flash production actually in combination with the loss of very talented flash artists tha had huge following in newgrounds early days.
Firstly, about 150 flashes are submitted a day, and more than half of those make it in. That was not the case 5 years. Back then it was slightly less per day but only about a quarter made it through the portal. This trend came from it getting it easier to make flashes that look better so even beginners look good and from spam voting, that's also let absolute garbage. I remember when even looking at the turd of the week was worth a look at because the stupidity of flashes like Dragonball N and the IsOnFire series was hilarious.
Secondly, we've lost... Legendaryfrog, knox, James Farr, Neil Cicierga, Sambakza is done with There She Is, Adam Phillips is starting his Bitey movie, David Firth, and countless others.