Ark server operators create a secondary "VTXO" system which eliminate the need for the bidirectional channel setup that LN has.
LN suffers from liquidity lockup: bitcoin sits on channels mostly inefficiently until a payment is initiated.
If your VTXO expires, you cannot unilaterally exit anymore so users are incentivized to move the money from time to time, avoiding inefficiencies of dormant liquidity.
I may be wrong but that's my curent understanding.
