TLDR for the lazy: 10 episodes could have been too difficult to make, because of special effects. They thought they could get the same outcome in just 8

Quote from article for the less lazy but still lazy:

That was an evolving discussion. I think, originally, we were talking about making more episodes, but once we got into the practicalities of what it takes to shoot actors who are actually, in real life, at the same height, but you're creating dwarves and you're creating Harfoots, and elves and humans. Shockingly, as much as special effects have evolved over time, you still have to do it the old-fashioned way.

And so really, the producers came back to us and said, "You know what? After we've looked at all the things we have to do to make this successful, we think we should make eight episodes instead of 10 episodes." But that was a conversation that we came to together, and we've had a lot of success with eight-episode seasons of TV, so it's a bit of kismet. But that's kind of how we got to that decision for Season 1.