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.