Why can't I use toaster oven instead of real oven?
Possible reasons:


Horrible temperature control - this is simply a problem of cheap
construction.  One could theoretically build an excellent toaster
oven with precise temperature control, but why would you? 
Low
thermal mass -  If you get an oven to 400F and open it for long
enough to put in food, the result is an oven at very slightly less
than 400F, and which will quickly return to 400F.  Do that with a
toaster oven, and who knows what temperature you'll get, or how
quickly it will get to your desired temperature (if ever, see #1). 
This is not a slam at toaster ovens, this is just due to them being
approximately 2% of the volume of a standard oven.   
Closeness to
elements.  It's easy for toaster ovens interiors to vary by nearly
50F from middle to edge, simply based on distance to elements


Now don't get me wrong, I once made some kickin' Oysters Rockefeller in a toaster oven, when I thought that they would be the difference between serving an appetizer and being allowed to serve breakfast the next morning (wink, wink).  If you know what you're doing, and watch carefully and continually, a toaster oven can be an excellent source of high(ish) direct heat.  But they are extremely difficult to control, and extremely unforgiving of errors.  I can't think of a circumstance where a toaster oven would be my first choice of cooking tool (and that includes microwave, campfire, plumber's blowtorch).