Ahhh - probably because we are trying to install a GPRS OS on a CDMA phone?
I modified my alx files so that it included 'CDMA' along with 'GPRS' (platform.alx, blackberry.alx). In addition, I changed the platform.alx OS node to:
<os radio="CDMA" series="8700" Colour="True" Theme="Normal" JVMLevel="1.0" Memory="Large" VoiceNotes="True" KeyboardType="Qwerty" SystemSize="normal" MMS="True" Sound="Tunes7130c" Bluetooth="True">CDMA/rim8703c.sfi</os>
and then copied the CDMA/rim8703c.sfi folder/file from my own OS installation for 4.2 to the 4.5 folder...and of course, deleting the vendor.xml file.
While this made it possible to install the OS on my Sprint 8703, it did not work at ALL. The device kept taunting me with the hourglass of dispair, and I am now downgrading back to 4.2.