Because the hardware interrupts are different, and the principle developer of the Overflow software has now stopped the project. Maybe someone else will come along and carry on with the work he stopped.
Overflow software essentially extends the functioanlity of the machine, that's all. In its original form, it was not intended for hacking channels, although someone (much to the chagrin of the author) did patch one of the versions to emulate Irdeto. Recent tweaks ensure that sufficient info is passed through the CAM to the card to allow a suitably programmed card to open Mediaguard and Viaccess channels as well as Irdeto (&Betacrypt).
The kinds of things that Overflow does over and beyond the standard Nokia firmware, are mainly aimed at the technical hobbyist or the obsessive fiddler. Things like: full display of PIDs/NIDs/etc; determination of exact symbol-rates; customisation of the front-LCD alphanumeric display; alternate fonts for the banner text; tweaking of video and audio settings...
For the casual TV watcher it is not recommended, not least because it reassigns the functions of the remote control keys (don't think the family would be too chuffed about that if it was their main receiver...).
But there is something deeply satisfying about having a machine freed by hobbyists from its shackles, rather than deliberately crippled by the manufacturer and/or provider(s) to their own insidious ends.