Because they specially make 70 sat doge buying order, while market price is 50-55, and much people send a huge amount of doge to exvo and sell it at 70 sat, so when they want to withdraw coins from exvo they should to use BTC for withdrawal, doge sellers can't to use an alts for withdrawal because on the exvo an alts has a small volume and high price. Now money blocked on the exvo and the exvo don't want to withdraw people's BTC.