How many providers do you have? Unless you're really abusing ambients, it really shouldn't be very many. If you really wanted to, you could create your own

PreviewProvider

that wraps all of them up into a single provider, but I wouldn't recommend it because it makes your previews less free-standing. One of the points of Preview functions is that they show how to use the widget, including which dependencies that widget needs. The fact they are free-standing with no implicit values provided is very much a feature, not a bug.