No reason I can think of, the D430 is more powerful than the Mini 12. The mini 12 and Mini 9 are designed to be the Volkswagen Beetle of laptops whereas the D430 was designed to be a laptop that could perform the same tasks and run the same apps as a bigger laptop. Just in a smaller size.
If you're thinking more along the lines of an upgrade you should probably look to the E4200 Latitude. Which is the model that replaced the D430.