Because the wallets used would be disposable, single-use. I'm assuming a launderer savvy and paranoid enough to use anonymizers of one kind or another to connect to the wallet provider. They'd want to avoid reusing wallets to lower the risk of correllation analysis blowing their operation. Each transaction would therefore involve creating at least two new wallets.

Now, an automated system for something like this would probably stand out  other users, at least in sufficient volume. I don't know to what extent this is happening, but I do believe it is being done - it just makes sense. My point is, with activities requiring the constant creation of new wallets, you'd expect the number of wallets to keep rising even if the number of users did not increase.  Rate of change may be a more interesting statistic to look at.