The risk of hitting a second-best hand and losing a lot.
Weak Ax, small flushes, and the low end of straights are typical. When you hit, you pay off a stronger hand in a big way. Example. Top pair with A4o on A♠ 9♦ 5♣ pays off AK and AQ in a big way. This is almost always what's behind a low-EQR hand.