Yes it's a shame Mike but in reality Illustrious would not have been a good candidate for preservation, just a money pit. The size of the hangar is quite small for the size of the ship (which is why HMS Eagle was chosen to stay in service instead) and the rest of the ship is a rabbit warren of small compartments which would be a nightmare to convert for general public access if it were possible at all. The chances that she could pay her way would be pretty minimal and no public subsidy could be expected in the current economic climate. She has not participated in any major battles and therefore doesn't have a high public profile as, say, does HMS Belfast. Mooring opportunities and costs for such a large vessel also present a big problem.
Existing preserved ships struggle to pay their way, even those in Portsmouth and Chatham dockyards and their future is always precarious. The general public doesn't appreciate our shipping and naval heritage as it once did, and is often quite ignorant of it so ship preservation is always an uphill struggle. Not the way we would want it perhaps but that is the reality.
Colin