That would've been a valid argument if army hadn't put forward 15 ton limit out of thin air after the system was already developed. Pure and simple sabotage
Again, absolutely valid concern regarding tweaking of GSQRs to favour a certain OEM but the example you chose is wrong.
Tube artillery is strictly a tactical weapon which are subjected to way stricter scrutiny. Whereas Pralay is a Theater Ballistic Missiles which is closer to strategic weapons than tactical; and here the focus is on just one or two primary objectives instead of a detailed requirement.
For example; lightweight, highly automated, having NBC protection, shrapnel protection, auto-loader, MRSI capability, extended barrel life, active cooling...can be just some of requirements you've for an artillery. You miss any one of these and you're automatically disqualified. Don't know how much you're into firearms, but there is this one famous incident of a firm by the name Robinson Armaments getting disqualified from the US Army's SCAR trials because they had missed to ship a Blank Firing Adapter with their gun...a tiny piece of bent steel with a screw; that's it.
For strategic missiles the requirements are pretty straightforward; X km range, Y kg payload and Z m of CEP.
Also you've the fact that you can't just buy a ballistic missile in open market because of MTCR treaties. Accepting whatever your local MIC churns out is the only option you have.
You're suggesting heavier is better in terms of performance. I would disagree. I don't think pralay is better than iskander.
Yes, heavier is indeed better in terms of performance when we talk rocket motors from the same time period. Iskander weighs 3.8t in total...and just the propellant alone on a Pralay is 3t. Which is going to have better performance?
I'd really love to hear from you how a 500km ranged Iskander is better than around 1,000km ranged Pralay.