According to Vatican correspondent John L. Allen Jr. in his book All the Pope's Men
, one reason that the Vatican doesn't sell off such treasures is that the Vatican doesn't believe that the treasures are the Vatican's to sell. They are considered the patrimony of mankind, entrusted to the Vatican for safekeeping, and cannot be sold or borrowed against. Indeed, the Vatican values them at one euro each for purposes of internal bookkeeping and spends a great deal of money to preserve the treasures for future generations.
If such treasures were sold and the money given to the poor, that money would soon be gone and mankind would be culturally impoverished by the loss of such artifacts into private hands. Besides, it is not offensive to religious sensibilities for a Church to maintain such beautiful treasures for the glory of God. If the Temple in Jerusalem could be richly ordained in order to glorify God and to inspire human worship, the universal Church of God can be richly ordained for the same purposes.