Because a GSM phone won't work on a CDMA network.
The bcm43xx driver is being developed as a clean room design, based on the reverse-enginered specs, created by another team. As it seems now, the bcw driver in question might just be removed from the OpenBSD source."[...]We believe that you might have directly copied code out of bcm43xx (licensed under GPL v2), without our explicit permission, into bcw (licensed under BSD license). There are implementation details in bcm43xx that appear exactly the same in bcw. These implementation details clearly don't come from the open specifications at bcm-specs.sipsolutions.net or bcm-v4.sipsolutions.net.
Those who can't write, write manuals.