Technically, they DID... The Atbara is a major seasonal tributary responsible for much of the volume of the annual inundation, and they did expand there. During the New Kingdom, there also appears to have been intermittent Egyptian overlordship over the land they called "Irem", around the confluence of the Blue and White Niles at modern Khartoum.
However, they didn't expand beyond the Khartoum area because there wasn't really anything to conquer... The local population at the time was incredibly sparse and nomadic - why bother with that when you have to keep a firm hand on the wealth of Nubia? And if they had gone farther south, they would have encountered unfamiliar, swampy environments (again, with pretty much nothing worth conquering)... Besides, the White Nile ceases to be navigable around modern Juba, and the Egyptians wouldn't have bothered exploring the White Nile if they couldn't sail it.
The Egyptians did sail very far south by sea, however, in order to trade with Punt... And sometimes Puntite merchants followed an overland route to the Nile, and the Egyptians met them at the more southerly extremes of their empire.