They were never going to be long tern on Youtube. They started around their six form/college (UK terms) age with added year out and early university years then life starts happening and their priorities changed.
They were cute, did random nonsense, the quality of their videos were good (decent camera etc) so they easily found an audience.
But things move on.
Charlieissocoollike (or however it was spelt) used to have a similar massive following (cute with random content) and then he moved on to other things as a career started to take hold.