Not being a maker, I'm no expert, but from what I've read, the "dimples" (in the centre of the tone-fields) are one of the main reasons for the difference in sound.
I seem to remember reading something about them "controlling unwanted partials that Handpan players find undesirable" (but can't find the source off-hand). Traditional Steelpan do not tend to have these dimples.
There are probably many other factors too. Such as the resonating chamber on the bottom that adds a sort of natural echo to the sound. And then for makers who nitride, the shells have a more ceramic quality - which also (to my knowledge) effects the sound.