If you have any sense of smell, and you fuel both a road car and an airplane which uses 100LL, you soon notice that compared to 100LL, road car fuel stinks to high heaven. Quite simply, it smells "nasty" when compared to 100LL.
The reason for this is that, compared to road car fuel, 100LL is more tightly regulated. There is a single standard (ASTM D910 in the USA) to which Avgas has to conform. This standard has been in place for a long time, and there are no local exceptions to the standard. All 100LL brewed in the USA has to conform to this standard. As a result, refineries do not adjust the composition of 100LL when they brew it the way that they adjust the composition of road car gasolines. For example, ASTM D910 does not allow the addition of ethanol. It has no "summer fuel" rules, and no state-specific rules either.
Avgas is comprised largely of alkylate, which is a high-octane feedstock produced in an alkylation unit in a refinery. Here is some of a Wikipedia entry about alkylate:

The product is called alkylate and is composed of a mixture of high-octane, branched-chain paraffinic hydrocarbons (mostly isopentane and isooctane). Alkylate is a premium gasoline blending stock because it has exceptional antiknock properties and is clean burning. Alkylate is also a key component of avgas. The octane number of the alkylate depends mainly upon the kind of olefins used and upon operating conditions. For example, isooctane results from combining butylene with isobutane and has an octane rating of 100 by definition. There are other products in the alkylate, so the octane rating will vary accordingly.

Raw alkylate can have a RON (Research Octane Number) of up to 100, but more commonly it has a RON of between 90 and 94. Often the refinery will add some reformate stock to adjust or optimize the octane rating. Finally, a minimum amount of tetraethyl lead is added to bring the octane rating up to the level required in the standard. The standard specifies a maximum lead content, but most 100LL contains a lot less lead than the allowed maximum.
The final result is not only a high-octane fuel, and, because it is usually blended from only two feedstocks, a chemically very pure fuel compared to the road car gasoline that flows out of the pump at your local filling station. That fuel is a blended base stock from a refinery (possibly containing dozens of different feedstocks). To that can be added ethanol, and the garage operator's own proprietary additive package. The result is a reasonably high-octane but also complex (and usually foul-smelling) brew.
The bad news is that alkylate and reformate are both premium gasoline feedstock. Nothing else comes close to them for sustained high octane ratings; therefore the cost of alkylate and reformate feedstocks is high, especially in the summer months, when more of it is used in road car gasoline brewing to meet summer gasoline standards. There is never enough alkylate and reformate to go around in the refining system.
100LL is only produced in batches to meet anticipated demand, it is not produced continously unlike road car gasoline. It also requires purging/cleaning of the refinery hardware used for brewing after use, because of the lead contamination. This, plus the cost of its dedicated distribution network comprised entirely of trucks and storage tanks (no pipelines) gives the three reasons why 100LL will always be more expensive than road car gasoline:

1. Use of premium feedstocks (alkylate and reformate)
2. Special refining procedures based on occasional on-demand production
3. Dedicated distribution infrastructure