Why is belly putters better for putting then conventional ones?

Like most things, for some, they are better. For others, they aren't. Technically, anchoring a club to the belly, chest, chin, etc. eliminates breakdown of little muscles in the wrists. Theoretically, it would make sense that it would create a truer and more consistent stroke over time. But those opposed to using it cite loss of feel, which is critical when putting. Of course, no stick can overcome failure to read greens properly.

Incidentally, European Tour legend, Colin Montgomerie, who has used alternative putters quite a bit in his career, is on record as saying that they should all be abolished. He is so convinced that they provide a distinct advantage at the world class level. Yet, no one has ever won a major championship with anything but a traditional putter. Sergio Garcia was aiming to be the first-ever at the British Open.