They probably simply haven't caught up with demand yet. There has been a lot bigger demand for Tether since around January when the Bitcoin price spiked. That is the risk with a centralized coin like this though.

I prefer decentralized versions like bitUSD, Steem Dollar, or Nubuts, but they don't have near the liquidity or price parity with the USD that Tether has. They unfortunately never gained enough popularity/liquidity to compete with Tether.