Because it's costly and unnecessary for a business to cater to every specific need. How many people even know what an MKV file is? And if they did how many people have the bandwidth to download one?
There are so many options available to people now (theatres, DVD, Bluray, iTunes, Amazon, Netflix) that people are resorting to insanely specific demands (MKV files, availability while in theatres, all I can watch for $5 per month, the technology used to stream the content must be one I like) in order to justify their piracy.