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One of the quirks of nature is that most metals have the same strength-density. That is for a given weight, the breaking strength is the same regardless of the material. So while aluminum is lighter, it is also less strong, so more of it has to be used.
So why do they use it on airplanes? Because some parts of the airplane aren't stressed members - and in those locations it makes sense to use a light material. In fact the Russians used steel parts in their aircraft to great effect.
Besides, the bonding between aluminum and rubber is very poor. Bonding rubber to steel is a 100 year old technology - well understand and very reliable.
Carbon fiber / carbon nanotubes? At this point very VERY expensive.
Weight of rubber compounds (I think you mean density)? Not a lot of manuevring room there. The compounds have to work in each area of the tire and that presents an interesting "weight vs suitability" problem. For example: The rubber in the bead area has to seal against the rim, and at the same time properly bond to the steel in the bead wires. These seem to be contradictory things.
Nevertheless, technology moves on and there may be some breakthroughs later.
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