What everybody else said... plus, bells are just plain too soft. My experience is that even people walking alone without earbuds usually don't hear the bell that came on my commuter bike.

I got a pretty loud squeezy horn, which people walking alone without earbuds almost always hear. (People who actively are shutting out paying attention to their surroundings - talking to others either present or on the phone, or listening with earbuds - still won't hear my horn.)

I don't have a loud voice, but I can still yell louder than my horn when I choose too. But there will always be people who ignore anything softer than an air horn (just as there are people on the road who ignore ambulance sirens), and as we've talked about before here, when you're on a MUP you just have to be prepared at any time to take the grass and/or come to a complete stop.