Toyota MR2 and MR-S are like this. I've heard that they don't handle
particulary well (not enough weight at the front of the car).

Poor weight distribution leads to lousy handling.

A Ferrari is actually 'mid-engined'.

Graham

The biggest reason is packaging.

You could build a rear-engined car with perfect handling (and no,
Ferrari is not doing that; the cars you are think of are *mid*-engined)
but when you put the engine behind the passengers, you use the best area
for storage.

Because of the aerodynamic and visual realities of automotive design,
you can build a higher and bigger trunk behind the rear window than you
could reasonable put in front of the front window. When you considerthe
room necessary to let the front wheels steer, it leaves you having to
make the front of the car even taller if you want to get the same
storage volume as you'd get in the rear.

Most engines -- even horizontally opposed arrangements -- are small and
low enough to fit in the space available between the front wheels and
below fender height; leaving the entire space between/above the rear
wheels/axle for storage. This can be maximized even more by making the
vehicle FWD. Then you don't need the space for a drive shaft,  
differential, and driving axles.

Yes, you can make more space by making the vehicle bigger overall, but
then it's going to weigh more, have more drag and cost more both to buy
and to operate; all things that people have been avoiding more and more.

Can't tell you if it costs less to produce or not - I've heard stories
both ways.  The configuration is much more forgiving when it comes to
slides and provides better traction in sloppy conditions.  The lack of
a transmission allows all of that stuff we love to be crammed into the
same space.

There are some rwd cars still available, but not as many as before.  I
learned to drive when a few exotic cars like the Saab, DKW, Citroen had

front wheel drive and all others had rear drive.  Now the opposite is
true and to be honest I don't miss rwd any more than I miss unassisted
brakes and steering and non-airconditioned cars.

Rear engine was always considered an odd layout for passenger cars.   A
couple of them were pretty successful (though you might argue that the
VW Beetle and the first few generations of Porsche were, in a broad
sense, of the same design pattern) but here are my guesses at some
reasons why it never took over the mainstream.

A flawed first generation of the "unsafe at any speed" Chevy Corvair
probably had a lot to do with its lack of widespread adoption.
(Remember, it was supposed to be a mass market vehicle, whereas
Porsche's target market of sports-car connoisseurs was presumably able
to deal with, or even use to advantage, certain handling quirks.)    If
memory serves, the first generation of Beetles had a bit of a tendency
to spin too, and were tamed with some suspension improvements and a
longer wheelbase.

There is also the problem of how to cool a rear engine (mid engines are
even worse, but if you've already sacrificed the back seat in the name
of sportitude, what's a few more louvre vents and hoses and so forth?).
   Really successful rear-engine cars, as far as I can figure out, all
had boxer or at least low-rising engines and were usually air-cooled.

And then there's the trunk.  People can and do put the trunk in front,
but that limits both its size (unless you are willing to accept
dreadful aerodynamics) and the ability to put extra stuff in there and
drive around with the lid up.    Remember, the front wheels have to
have room to swivel as well as to go up and down.

In a big vehicle, you have to run the control cables and linkages allll
the way back there without sacrificing acceptable feel and reliability
-- a more or less solvable problem but not the obvious way to go.

Although I'm not sure when it kicked-in as a design priority, having
the engine (and, lately, transaxle) up front, as well as the cooling
system and so forth, doubtless makes it easier to absorb energy in a
frontal collision.

A rear engine or "pusher"  configuration is the premium setup for buses
and large RV's and has been for a long time, for instance, but they can
have the "trunk" slung underneath.   Of course, buses and RV's are not
usually flogged around hard enough for people to care about polar
moment and trailing-throttle oversteer.

Why it is generally not employed for delivery trucks, I dunno.
Probably because rear access to a standard loading dock, engine and
transmission accessibility, and cooling system location amount to one
thing too many.

I missed the rear-engine part of your question.  A rear-engine
rear-wheel drive combination is possibly the worst configuration for
most drivers.  As a group such cars have potentially dangerous steering
characteristics in the hands of the average driver and the cars tend to
be very sensitive to wide-winds.  Take the engine/transmission
combination and move it to the front and you have a much better chance
of creating a car with superior traction, steering and tracking
characteristics.

The old Renault, Fiat and VW cars were not designed for the highway
travel they were often subjected to and their handling characteristics
could be downright scary.  The later VW Variant and yes the Corvair
were better handling cars, but they too suffered from having too much
weight in the wrong end of the car.

Thank goodness we have for the most part gotten away from rear wheel
and engine cars.