The same way cash isn't solely a tool of criminal interests. While privacy can be used to conceal illegal activities, that's far from its only use.
There are a lot of law-abiding people who favor privacy-focused services for perfectly legitimate and legal purposes (DuckDuckGo over Google, TOR over regular browsing, "private browsing" or "incognito mode" over normal browser operation, etc.), and I see no reason to assume that wouldn't extend to money as well. Actually, I'm surprised that it's taken this long for a privacy-oriented crypto currency to arise, given how much more prevalent (and potentially devastating) financial monitoring is, compared to the communications monitoring that's making headlines over the past few years.
And honestly, so what if Monero does become the defacto currency for crime? The fact that cash is the favored currency of the black market and criminals the world over now isn't a serious argument against the use of cash, so why should Monero be held to a higher standard?