I have been asked that question many times, especially in my driveway with a dozen bikes hanging in the open garage.
I just say "for money, unless you have something cool to trade."

That, somehow, tends to deflate the buyer's suspicion that I'm actually trying to sell it for money, which, of course, I am.

The 4-paragraph macroeconomic reason almost never goes over well. The buyers' eyes start to glaze as they slowly back up to their vehicle.

"Because giving it away makes my wife get mad" almost never works. I think the buyers consider that flippant, but some guys catch on and offer $1.

"To meet girls" almost never works with female buyers. For some reason, they think that's creepy.

"I have others I like better," or "it doesn't fit" seem to work well, and generate a measure of empathy with certain buyers.
They don't need to buy it, either, but they sure know what I'm talking about.

"It's stolen" has not worked yet.

"It's not a Centurion" works because most people cannot figure out the logic in that statement.

"It's not the kind of bike I like to ride" works pretty well, but generates low-ball offers in many cases.

"I'm getting into triathlons and I want a 12-lb aerodynamic carbon bike" always works.

"To buy meth" does not work.