Why didn't Thanos kill all the Dwarves on Nidavellir?

Because Eitri's life was given to him by Thanos.

Three hundred Dwarves lived on this ring. I thought if I did what he asked, they'd be safe. I made what he wanted: a device capable of harnessing the power of the Stones. And he killed everyone anyway. All except me. 'Your life is yours', he said. 'But your hands... Your hands are mine alone.'

Source: Eitri (Quote at Top of page)

From what Eitri says to Thor, he made the Gauntlet alone hoping to save his people and according to the wiki, it was also Thanos's own threat.

Thanos went to Nidavellir, asking Eitri to make an object that can harness the power of the Infinity Stones. The Mad Titan promised if he refuses to forge an object, Thanos would exterminate all of the Dwarves. Eitri successfully made the Infinity Gauntlet and Thanos proceeded to put it on and, ready to act on his quest to gather all six stones. Having successfully obtained the gauntlet, Thanos proceeded to massacre every dwarf, leaving Eitri by himself and telling him that his hands were his alone.

Source: Massacre of the Dwarves > Massacre

We know that Thanos is honorable so by sparing Eitri he kept his end of the deal by not exterminating all the dwarves, while at the same time prevent Eitri or any other dwarf from forging anything else to be used against him by casting his arms in metal and exterminating the rest.

As for anticipating Thor's actions, maybe it was an oversight but it was stacked against Thor to be rescued by the Guardians since he was just floating in space, had he not smacked against their ship they might have just left.

It's possible that Thanos just knew that Eitri would need help with the Neutron Star and it was likely that anyone who did would end up dying in the process as Thor would have died himself had Groot not made a replacement handle for Stormbreaker. If it was anyone else than Thor, I doubt they would have been able to keep the forge lit for the blade to have been made.

He should have known that a single dwarf can operate the forge by himself, should Thor appear there, Thanos is always a few steps ahead of everyone else.

Eitri didn't and couldn't operate the forge all by himself, Thor did a lot of the work with the help of Rocket and Groot even helped at the end. They simply wouldn't have been able to get the forge started again without Thor and Rocket.

As for why he didn't kill Eitri it probably comes down to a couple of things.

1. He'd rendered Eitri largely useless for forging having encased his hands in molten metal.

   Eitri: Then he killed everyone anyway. All except me. "Your life is yours." he said. "But your hands are mine alone."

   _{Avengers: Infinity War}

2. Eitri created the Infinity Gauntlet, he probably let him live in case he ever needed a new one and as thanks for doing it.

3. Killing all but one of the Dwarves already goes against Thanos' MO he probably didn't want to break it further by killing all of them.

So why kill all but Eitri? Stephen McFeely just states it's because "he's a ruthless SOB" but one could argue it's more down to wanting to keep the knowledge of the Infinity Gauntlet secret.

That was to show how ruthless Thanos is:

Unfortunately, Infinity War isn't particularly consistent with the idea of Thanos's relentless commitment to logically pursuing his goal. When Thor arrives on Nidavellir, for example, he discovers that Thanos has committed a horrific act of genocide. Where usually Thanos wipes out half of a people's population, as part of his insane idea of balance, on Nidavellir he had slaughtered them all. It's possible to argue that he had a reason for this - he didn't want knowledge of the Infinity Gauntlet's existence to be revealed, or for the Dwarves to forge a weapon that could defeat him - but that still doesn't track with Thanos leaving Eitri - King of the Dwarves - alive. As co-writer Stephen McFeely explains in The Art of Avengers: Infinity War, this was intended to show "that he's a ruthless SOB." - screenrant

Even when he left Eitri alive he made his hands useless, that's crueler than killing.