A hot concept in fast food marketing is home delivery of everything from pizza to hamburger to fried chicken. why do you think the demand for this...

Economists say that two of the reasons that demand can
change have to do with the income of the consumers and their tastes.  I would argue that
both of these could be causing the change that you
describe.

As people get to have more money, they are able
to afford more things.  Having food delivered to their homes could be something that
they were once unable to afford but can now buy with their added income.  As people
become busier, their tastes might change as well.  Being able to save time may become
more valuable to them.  In that case, they will be willing to spend money to save time
by having their food delivered.

I would argue, then, that
this change in demand is due to consumers becoming richer and busier and, therefore,
more willing to pay to have food delivered to their homes.