Why do diodes have a voltage drop?
The forward biased semiconductor junction takes a voltage level to be able to push the electronic charges over the P-N zone. Think of it as similar to how you have to "lift" each marble up to the table top from on the floor. In addition to the energy level difference needed to transport charge across the junction there is also a resistive part of the diode that drops some voltage as well. The resistive drop in the diode will be dependent upon the amount of current flow allowed through the junction.