There is one more, albeit minor reason why Unicom is text.
Range.
Think about it. 122.8 used everywhere in the US (keeping it regional, for the bounds of this forum). I'm at KPRX (uncontrolled field). I connect to a voice-enabled UNICOM channel on, say, rw.liveatc.net (one of the more popular voice servers we have). I notice some 75 other pilots on there as well, announcing their intentions at Rapid City, Boise, Erie, Duluth, Santa Fe, St. George, and Cheyenne). Absolutely none of them are near me, so why should I have to deal with hearing their intentions when I'm in Northeastern Texas? None of them are near eachother as well!
There is no way to control range on a voice server. And that brings up another point: What if I connected to the VATPAC voice server, while someone else at the same airport as me connects to the LiveATC server? I won't hear them, and they won't hear me. He starts his takeoff roll as I do, at opposite ends of the field.... You see what happens.
Text is at least covered by range, and does not provide that conflict that voice servers would.
BL.