Here’s the background on why Coinbase currently only supports limit orders for ZEC:
Here’s a summary of what @Beth and I are doing to address it:
Over the past few days, ViaBTC’s share has remained consistently under 51%, based on the data they provide for the last 1,000 blocks. Hopefully this trend continues. Coinbase has not specified an exact duration for ViaBTC to maintain a hashrate below 51% before resuming market orders; they have only indicated it needs to be sustained for “a period of time.”