Aviation fuel is dispensed at a limited number of places; it would be easier (or just as easy) to implement a higher aviation fuel tax than a higher auto fuel tax.
It's trivial to implement auto fuel tax - it's already in place in most of developed countries.
There's an auto fuel tax in the US. Increasing that from $0.184/gallon for gasoline and $0.244/gallon for diesel to say $1.50/gallon and $2.00/gallon would ensure massive losses for that party in the next two or three election cycles.
Increasing the tax on aviation fuel to $2/gallon wouldn't produce massive shifts in the next several elections, therefore it's easier to implement.