Actually I just found out a couple of reasons why:
David Klain wrote:
VATSIM is committed to serving the needs of all its members. VATSIM has a number of members who are text only for a variety of reasons (hardware, connectivity or hearing impairment). Those members are unable to hear voice transmissions. All VATSIM controllers automatically monitor text on their frequencies (the controller client software is built this way) and can provide text services when a pilot requires them. By its very nature, UNICOM is a frequency without a controller and there is no person "running" the frequency to ensure text-only pilots are aware of what is going on and getting the necessary information. As a result, VATSIM requires that all pilots transmitting on UNICOM transmit their information via text to ensure receipt by all concerned.
VATSIM is looking at a potential technical solution which would alert pilots when there is a text-only pilot on the UNICOM frequency and in range so they could use voice and only back it up with text when required. This is a significant programming change that would require changes to all of the pilot software clients (FSInn, Squawkbox, XSquawkbox, etc.) to support. If such a solution is possible is still under exploration.
http://forums.vatsim.net/viewtopic.php?f=84&t=38243And
Quote:
As things are now, you can not use voice on unicom. ONLY TEXT. The explanation is technical.
But if you are up to it read along:
The vatsim-servers (The ones you connect to) are all connected in one great network. So a guy
on the European server can still se a guy from The US even though he connected to a different
server. They are all connected. So when you write something on Unicom it's broadcasted to all
aircrafts near you. Regardless of which server pilots are connected to.
The voice servers are NOT connected to each other! So people on an America voice server
can't hear people on a European voice server. So both guys can be parked in the same airport,
but if they not are on the same voice-server (Unicom) they can't hear each other. However they
still can text each other.
http://www.vaccsca.org/pdf/pilot/training/unicom.pdf