They can get large numbers of them cheap, and their system is good at detecting and replacing bad drives, so why not use them?
Maybe it's just my social/business groups, but "because reasons" doesn't have to have a negative meaning. I use it as more a statement of fact: There are reasons for this.
If you're an individual person with a 12-drive ZFS RAIDZ2 with hotspare, in a file server in your garage or something, you might prefer to pay the premium to buy HGST, because what if you have a double drive failure during a three week period while you're on vacation out of the country?
https://www.google.com/search?&q=why+raid5+stops+working&ie=...
They have over 20 thousand HGST drives now, according to this year's report, which seems significant to me.
They continue to have lower failure rates than the Seagates.
That Seagate 10 TB was the singular standout in terms of reliability. Maybe it is worth checking out.
You can't argue with results.
Sometimes, personality dynamics cause certain approaches to work better than one might prefer. :)
I would be surprised if the number of drives they have is actually "statistically insignificant" \* but I haven't crunched the numbers.
https://en.wikipedia.org/wiki/Pearson%27s_chi-squared_test
\* By this I guess you mean that the null hypothesis of "they have the same failure rate" is not rejected.
Granted, I'm not a hardware guy so my takeaway could've been wrong, but that's what it looked like to me when reading them.