Because it just works.
In around 80 patches I've made on this so far, I tweaked osc volume exactly 0 times. Occasionally I would tweak noise volume, but there is no point to add to front panel control that is required sparingly. Amount of control on front panel is just perfect, because Oberheim did it right the first time.
Add second page for mod matrix? Yes.
Improve LFO? Hell yeah.
Option to assign computer keyboard key to flip front-back panel from? Very yes, and add such access to voice tweak pages and for voice mutes.
Front panel? It's pretty much great the way it is. Rearrange, OK. Clutter it with more controls? Hard nope.