[vhodges]

That's a good question and I don't have a nice pat answer for you.

It probably comes down to personal preference. It may work in a way that is better for you or you may like the way it looks or it may offer features that the others don't.

The world seems to like choices, we have many text editors, web browsers, languages and operating systems, restaurants, etc.

Note, I am also the founder of pennyminder.com so I think there is lots of room for both full featured PFM's as well as simple and unobtrusive ones.