Why are land auctions not in Linden dollars?

The subject says it all - why are all land auctions not in Linden dollars?

I don't know the answer, so I'm going to inquire further about this.

As Second Life has grown over the last few years we have shifted further away from a subscription based model toward one that is based on property sales and fees. These changes reflect the evolution of the SL economy to one based on land ownership.

For our business to be viable we need to charge US$ when we put large tracts of land online. These $ offset the costs of hardware, bandwidth and colocation. However, we do try to make sure that there is always some land available for L$ in the auctions, although for smaller, reclaimed parcels. And land is always available from other residents for L$.