In eastern Asia, where it is particularly popular, the figure usually represented is Vairocana as the Adi Buddha, wearing a crown and jewelry of the kind worn by Bodhisattvas.
This Buddha figure appears in both Hinayana and Vajrayana areas i.e. completely different religious systems. We can find couple explanation to why Buddha appears in Princely attire, who is otherwise invariably shown without any adornment.
The first explanation talks from the stand point of the doctrine of the Three Bodies. The Buddha appears richly bejeweled in the glory and brilliance of a transcendental being, when he manifests himself as Sambhogakaya to the Bodhisattvas in a supernatural realm. Sambhogakaya is a subtle body of limitless form.
The second concept talks about Buddha as world ruler entitles him to wear princely jewelry. This concept merges with Adi Buddha, the Absolute, Original, or All embracing Buddha, whose true self is invisible. His absolute nature, where all other Buddhas emanate or are made manifest. When we say all other Buddhas we are generally talking about the Five Buddhas of Wisdom.