Because the first drive I bought from them failed within a couple of months. Since then, I have only purchased Seagate. I did have some trouble with a drive that was three or four years old when I got a new PC. Other than that, they've given me no trouble at all.

I can't say that Western Digital is bad. It's been years, and it was just one drive. But it did change my mind about what drive to buy.

Hope this helps

Some of my external drives are quite old. I've never had a problem with either brand. I just recently checked all of the drives and they're all working. That said, I tend to buy Seagates now, based on what I've heard from others.

That, of course means little in the grand scheme of things, but goggling around, it depends upon whom you ask. Hitachi seems to have an edge over both of them.

They're cheap enough you might just buy two. Or three. And buy a few TBs of cloud storage.

I used to buy only Seagate but have a few WD as well.

I don't think I have ever had a Seagate outright fail on me.

That said this site has data for thousands of drives from the big manufacturers.
https://www.backblaze.com/blog/hard-drive-reliability-stats-q1-2016/

Looks like HGST drives fair best with WD pulling up the rear. That said different sized drives seem to have different failure rates.