Why does Irving mention Captain Kidd shortly before he introduces Tom and his wife?

The mention of Kidd and his hidden gold set up the theme of greed.... the reason Tom makes his pact with the devil.

Under one of these gigantic trees, according to old stories, there was a great amount of treasure buried by Kidd the pirate.

The black man told him of great sums of money buried by Kidd the pirate, under the oak trees on the high ridge, not far from the morass. All these were under his command, and protected by his power, so that none could find them but such as propitiated his favor. These he offered to place within Tom Walker’s reach, having conceived an especial kindness for him; but they were to be had only on certain conditions.