With their volume they can afford to keep draining from low volume trade. I mean that from their perspective your presence on exchange is burden. Think about it - they spend exactly the same amount of computing power to process your volume as they spend processing million dollar or any other trade. And they HAVE to support all customers equally. So the only logical way is to make those who generate less profit to pay more or get out.

Also AFAIK bittrex uses Microsoft platform to power their engine - that makes scalability really painful, so I assume they are struggling with common problem in IT - How to lower the load, without lowering profits. As usual users will be those who pay