Investigate the queries running against the instance. Identify why they're reading so much data

Edit: Sorry, originally read that as plan cache hit ratio, ignore what was here before...

Low buffer cache hit ratio means that queries are reading huge amounts of data, more than will fit into memory. Hence SQL has to keep discarding what's in memory and reading more and more from disk.

Identify the queries that are doing the most reads. See if you can identify why. Tune those queries to do less reads.