It is common knowledge that most illegal activities are conducted with cash. Those who wish to remain under the radar of the tax authorities prefer $100 bills.
Here's a second question: Why are large bill notes, which the majority of people and workers do not possess, still available in such large quantities?
Kenneth Rogoff states in his book "The curse of cash" that Robert Rubin believed 500 Euro notes could challenge $100 bills in the global underground economy, acknowledging in part that the ultimate utility of these notes is to facilitate shady transactions.
The European Central Bank (ECB) estimates that 20 to 25% of Euro currency is held outside the Eurozone.