Hi,
I think part of the reason is that on the basic DI scheme, you only have to comply with BS7671:2008 (soon to be 2011) and you are only testing/certifying what you have installed, and therefore have full control over, and which will (or should be) to current regs. You also don't always have to have 2391 for this.
To do PIR's you need a far greater indepth knowledge of not just the current regs but of previous regs as well, and all scheme providers insist on the 2391 for this.
The other part of the reason is because they will charge you for the extra assessments etc., and so is in part down to money.
You could still do PIR's, but just not under the umbrella of your current scheme provider, but this could cause other problems wrt if your client is say the local authority or insurance companies etc. who may very well insist that this activity is carried out under a 'scheme' banner so to speak.
Obviously you could not use your schemes sheets, letterhead etc. for this, or mislead your clients, and would still probably need the indemnity insurance, which you may have problems getting not being under a scheme.
Depending on whose scheme you are under, they may also take a dim view if you are doing PIR's not under them.
cross posted with NBP