In addition to the points made by jcbcode99 above, here are a few more issues to consider. Firstly, Diana was very young when she became Princess of Wales. You don't often see young royals wearing very heavy jewellery. Secondly, "bling" was not really very "in" in the early 1980s - and as a result, Diana was unlikely to want to be seen in much jewellery. Thirdly, the economic environment in the early 1980s (the early days of the Thatcher era) was certainly not condusive to displaying much jewellery. Fourthly, from relatively early in the Wales marriage, it was clear there were very fundamental differences between the couple. In such an environment, it is highly unlikely that the Queen would give the PoW more jewellery. Had the relationship been on a more stable footing, I am sure in time more jewellery would have been made available to Diana.
I hate getting dragged into the Camilla vs Di thing, but whilst a lot has been reported about the large collection of jewellery that Camilla has, a lot of the bigger pieces (other than the tiaras) have been presents from middle eastern rulers and not from the Queen herself. The rubies (often referred to as the breastplate), the sapphires, the emeralds (not yet seen publicly), and possibly, one of the diamond necklaces are from the Saudi's. The only "big" pieces of jewellery (other than the tiaras) from the Queen that we have seen Camilla wear are the collet necklace worn on Camilla's 60th birthday (and again for the Fench banquet) and the Greville 5 strand necklace worn in Kampala for the CHOGM.
Also, it is important now for Camilla to be perceived very much a future Queen, because like it or not, the day when Charles succeeds to the throne is not that far away. There still remains a question as to Camilla's eventual title, so the last thing the BRF would want is to show that they do not fully support Camilla.