Trend 6.4 of the Oxygen Group (Group VIA) Trends says that the thermal stabilities of the hydrides EH2 decrease down the column primarily because of the decreasing EH mean bond enthalpies.
Size matters[]
The atomic radii of elements increase down the column. Thus, because a hydrogen atom is so small, the larger elements will not be able to form as strong of a hydrogen bond as the smaller elements – top of the column – would be able to.