What is a simple way to explain why best range airspeed is below 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.