Furnace Replacement - We are replacing our old furnace and I am wondering why we have to replace all ducting? All quotes I am getting include price for new ducting.

How old are they? Newer ones are often smaller and the flexible ducts can go through warmer areas and shorter routes, and of course everything is insulated. Ask the contractors who gave the quotes what their reasons are.
There might be nothing wrong with the ducts per se, but their size and placement might be inefficient; e.g. if they run around the perimeter of the basement.

Only reason to replace all ductwork would be due to extremely incorrectly sized ductwork. However, most likely the ductwork they are referring to is simply the plenum and return that actually hook to the furnace. Newer furnaces are much smaller than older ones, and require new ductwork to convert from the old furnace to new furnace. This is simply ductwork to hook the new unit to the existing ductwork. Can you tell us what the quote is for the "new ducting"? If the "new ducting" is only a few hundred dollars, its just what I said above, if its up in the thousands, they are for what ever reason replacing most all of your ducting system.

It could be a suggested part of the installation of the new furnace because of higher efficiency.
Older duct work in homes tends not to be insulated and made of tin or galvanized sheet metal that sheds heat like crazy. The new duct work is probably a flexible tubing type of ducting that provide better thermal retention therefore transferring more heat to the home. If you are shooting for better efficiency to save money, the new duct work is probably a very good idea.

I'm going to respectfully disagree with Handyman2007. Metal duct size for size will carry air more efficiently than flex. Depending upon where the duct is, conditioned or unconditioned space, it may not even have to be insulated. The reason for replacing ducts when replacing a furnace is quite often the existing ducts, particularly the returns, are too small.

If the flex ducting is sized properly, there will be no difference and will still retain heat longer and more efficiently. With metal ducting you also have hard angles 45 degree ands even 90 degree. That you will not have in flex ducting. Every angle means less effective air from reaching it's desired location.
A recent commercial installation of heat and dehumidification in a food processing plant revealed a 35% increase in overall efficiency of air movement and temperature transfer.

Residential vs commercial. Two completely different monsters.
In residential, sheet metal duckwork is still the norm. A good tinner will not use sharp offsets (45s 90s, etc) but will build radial offsets (rounded offsets). However, in most residential settings, even hard offsets work just fine. We use sheet metal ducting for main trunk lines, and six inch round snappy for register supplies in most cases with no issue (as long as all is sized correctly.) Most newer furnaces are either two stage, or modulating and can be set up to function properly and to the rated efficiency with most existing duct systems (within reason).. Return is usually the big one that gets overlooked and in many cases extra return needs to be installed. However, with out us being able to see the home in question, we are basically just throwing out guesses. Since marie has gotten multiple quotes, and all included "ductwork" I say she just needs to leave this up to the pros that are giving the quotes after seeing the system first hand. Pick the one you feel most comfortable with and good luck on your project.