I'm not a drifter (at least not that kind) so I don't claim to know for certain, but I suspect there are three reasons why people chose to make Impreza's RWD for drifting.
1. Most other small, affordable cars are FWD and converting a FWD to a RWD is substantially more challenging than converting an AWD to RWD.
2. Compared to most other RWD cars the Impreza is relatively affordable, even if you factor in the cost of converting it to RWD.
3. Its relatively stiff chassis and low centre-of-gravity give it good handling characteristics.