Hypothesis: The reasons to sell $L are stronger than the reasons to buy $L.
Example:
The main reason to buy $L on Lindex would be to purchase land.
The main reason to sell $L on Lindex would be to pay tier.
Buying land, for most SLers, does not happen very often.
Paying tier on the land you bought happens every month and is in $US.
Result:
The volume of "Tier Paying Selling" would be larger than "Land Purchasing Buying".
This imbalance would mean the $L price would go down over time?