Perhaps she didn't ask! I'm sure that HM would not have refused had Diana asked to see what was available from the items that were not in use.
She didn't need more than the two she had. At the time of her marriage, she had exclusive use of the Spencer tiara, which her grandfather had actually left to her brother, Charles, after his death in 1975. Since Charles was unmarried at the time, she continued to use it until he asked for it to be returned.
The Lover's Knot Tiara was rather heavy. Although Diana said she loved it, it gave her major headaches and she preferred not to wear a tiara at all, which she didn't unless absolutely necessary.