#3 Lack of push
When Rusev Day was the biggest that it had ever been, if WWE had pushed them, then their popularity could have been even more astronomical, something that cannot be considered a possibility today.
While they were a popular tag team who finally received their shot at the titles at Hell in a Cell, their 'oomph' factor, which they had towards the beginning of their run was long gone. No longer two of the hottest stars in the business, they were still cheered, but the initial lack of push had harmed them.
Also, WWE seemed unlikely to push the team in a major way, while Aiden English was still in the team. They seem to view him as more of a comedy act, than a real competitor, and the push just was not coming.