Because she had struck a bargain with him. Quoting the Wikia:
By the time the Black Pearl reached the coast of Shipwreck Island,
Barbossa stood on the ship's forecastle, conversing with Tia Dalma,
regarding their bargain. The mystic accused him on intending to betray
her and therefore reminded him on that he was brought back from the
dead by her power, and also, demonstrating her abilities by briefly
decomposing Barbossa's right hand, cautioning him of his fate should
he fail to keep his end of the bargain. Barbossa in turn reminded her
that she, secretly the goddess Calypso, was the one who needed him;
she resurrected Barbossa so he could help retrieve Jack Sparrow from
Davy Jones' Locker and summon a meeting of the Brethren Court, since
only the nine Pirate Lords had the power to free Calypso. To avoid the
risk of the goddess reaching an accord with someone else, Barbossa
ordered Pintel and Ragetti to lock Tia Dalma in the brig. Barbossa
remained on deck, glancing at his right hand while contemplating his
The Wikia also provides the relevant dialogue:
Barbossa: "I do not renege on a bargain once struck. But we agreed on ends only.
The means are mine to decide."
Calypso: "Caution, Barbossa. Do not forget it was by my power you return from
the dead. Or what it mean if you fail me."
Barbossa: "Don't you forget why you had to bring me back. Why I could not leave
Jack to his well-deserved fate. It took nine Pirate Lords to bind you,
Calypso. And it'll take no less than nine to set you free."
Although to be honest the same Wikia page somewhat contradicts this in an earlier paragraph:
With the death of Hector Barbossa, the Pirate Lord of the Caspian Sea,
came a prime opportunity. Tia Dalma/Calypso needed all the Pirate
Lords in order to assemble the Brethren Court, so they could release
her from her human bonds. And so, through undetermined circumstances,
she resurrected Barbossa from the dead and tricked him to summon a
meeting of the Brethren and retrieve Jack Sparrow, who was taken to
Davy Jones' Locker by the Kraken.
It seems odd that she had to trick Barbossa into doing what she needed, when his task was pretty specific: rescue Jack Sparrow and convene all the Pirate Lords.
Thus the exact circumstances of their deal remain unresolved.
Why the other Pirate Lords didn't recognize her seems to be a plot hole as well. Calypso does not seem to have powers to have changed her appearance, so her guise as Tia Dalma does seem flimsy.