I think purpose of Pralay maybe to incorporate all the latest technologies/techniques mastered over decades - Prithvi is 40 year old tech now (in terms of materials, engine, propellants, guidance etc.).If Parlay is more or less Solid fueled Prithvi then what is the need to build it.
Further, the fact this is single-staged also hints at the desire to produce these cheaply and probably in large quantities. Shaurya & other two-staged Prithvi will definitely be more expensive (not to mention complicated) with two engines, binding/separation mechanisms etc.
1 tonne payload capability may also help Pralay to carry some kind of glide bomb or short-range missile to extend the terminal range , now that DRDO does have the capability to do so ( just speculating here).