Question

Why can't there be a number lower than absolute zero

Answer 1

Absolute zero is not about numbers.  It's about temperature, and the
motion of molecules in gases. 

You know that the temperature we feel with our skin is the result of the
average speed of all the tiny molecules zipping around or vibrating in
the solid, liquid, or gas.

The faster they're all moving, the warmer the substance feels to us. 
The slower they're all moving, the cooler the substance feels to us.

When molecules slow down to zero and lose all of their kinetic energy,
that temperature is what we call 'absolute zero' ... if they're not moving
at all, then they can't move any slower.

Answer 2

Temperature lower than absolute zero is not possible. Lower than absolute zero a gas contain negative pressure and volume which is not possible. temperature is the energy created by moving atoms absoulte zero is a complete stand still you can make particles move to the end of the universe but the coldest temperature will be stuck at the point where atoms are motionless