It is mostly because Inteleon is outclassed by Sweepers such as Rotom-W and Special Dragapult. Inteleon overall has a very shallow movepool and usually relies on Choice Specs to do heavy damage. As a result of its shallow movepool consisting of Air Slash, Ice Beam and Hydro Pump, it struggles to take down common threats like Ferrothorn, along with other Walls and even sweepers. In addition, it is very frail. This usually results in a 1 or 2 Hit KO for the opponent. Also, it has basically no coverage for its weaknesses. Sure it has Ice Beam, but if you don't kill that, it is going to kill you. So, its shallow movepool and it being outclassed by Sweepers with better potential along with its Horrible bulk led to it being in RU.
Hope this helps!