Because, with some (most) heavy jets aircraft, the main landing gear is well behind, and lower, than the pilots eye level.
With the L1011, as just one example, the MLG is 79 feet behind, and 38 feet lower than the pilot (in the landing configuration attitude) so....IF you get too low, sufficient approach light clearance is not guaranteed.
Therefore, there is a minimum threshold crossing height (TCH) established...in the case of the L1011, it is 42 feet.
The TCH is normally printed on the specific approach chart.