Balance issues, more than likely. But back in the day, some people wanted it so that you could pick across powersets within the Archetype primary/secondary. For example, if you were a tanker, you could pick your tier 1 power from Stone Armor, and then pick your tier 2 power from Ice Armor, etc. And interesting idea, but probably just as problematic.
That I'd like, i've always wanted to see an "Elemental" mage type having a mix of fire, ice, water, earth etc
Also can't remember who said it wasn't but Sent is 100% a score creation.