What is a simple way to explain why best range airspeed is below normal cruise airspeed? Without using complex mathematics, please explain why best range airspeed is less than normal cruise airspeed.
Normal cruise is a compromise between being fuel efficient while getting to your destination at a reasonable speed.  It all comes down to drag; the faster you go the more parasitic drag you pick up and economy suffers.

Just like in your car - if you wanted to squeeze max efficiency (range) out of a trip you could drive at 55mph on the freeway.  In fact you could even make it law!  But, people are generally willing to burn a little more fuel to get there faster.