In the past, they've said that availability and price are their biggest factors, as long as reliability is ok. Their architecture allows for a pretty high failure rate, especially if pre-failure indicators allow them to migrate data from the drive before it fails.
Moving to any particular drive probably indicates bulk purchasing availability at reasonable prices more than anything else.
They go over this in the article: below a certain failure threshold, which they don’t appear to have hit, it’s still cost effective to go with the less reliable drive if it’s cheaper.
Moving to any particular drive probably indicates bulk purchasing availability at reasonable prices more than anything else.