Firstly, Thanos is one of the most powerful beings in the universe, the same with Hela. Thanos is known to possess a superhuman power as Hela with exception of magic. She draws the power directly from Asgard and able to live as long as Asgard lives which made her too powerful with God-like power and able to wield a strong, dark magic: exactly how she is able to stop the hammer and break it to pieces by magic. She is very smart as she can even tell that some of the relics kept in the vault are fake or weak.
However, the closest magic Thanos wield was the infinity stones. It's much powerful when they are together, which can wipe out whoever the holder, Thanos wants, but he's no match for Hela's magic.
Secondly, Hela is prepared to what coming to her when she faced Thor and Thanos didn't. Thus, he can't stop the axe from hitting him. "You should have gotten to the head." That's what he said to Thor. The powerful Stormbreaker able to cut through anything thus only severely injured him as he has a superhuman strength.
He instead, didn't waste any time and finished his own plan, meaning it's worthless for him to use the time stone to go back only so he could shield and break the Stormbreaker. He has a superhuman ability, able to heal himself anyway.
The only reason Mjolnir is only for the one who is worthy is that Odin put a spell ("whosoever holds this hammer, if he is worthy, shall possess the power of Thor") on it after he banished Thor to Earth and threw the hammer away.
But I don't think this is relevant as even as such enchantment to be cast on the Stormbreaker, it'll only become heavy and stationery if only the not so worthy touch it, but in this scene, Thor touched the axe as he shoved it deeper to Thanos' chest.