You may have an old car which is still in very good condition and perfectly roadworthy. Unfortunately however it will have been built to the safety standards which were acceptable at the time. Since then there have been enormous advances in driver aids such as electronic stability control (which is now mandatory on all new cars built within the European Union), seat belts and seat belt reminders, airbags, lane control systems and automatic emergency braking systems designed to prevent, or at least reduce the impact of, collisions with other vehicles. These measures have made cars infinitely more safe than they were even just a few years ago.
If we add to this the fact that cars are now intrinsically safer, with crumple zones designed to protect both occupants and third parties, with better steering, braking and roadholding, it is hardly surprising that so few insurers are prepared to cover older cars.
There is also a practical reason for this reluctance. Most car insurance nowadays is bought over the Internet through price comparison sites. In order for these to work efficiently it is necessary for the insurance companies to have a finite number of cars that they are prepared to insure, and also statistical data about the likelihood of these cars having accidents, and the approximate cost of claims. They cannot do this for every model and every make of vehicle so if you want to insure a car which has been out of production for some considerable time it is unlikely that an automated system will be able to provide a quote for it.
Nevertheless there are insurers out there who are quite happy to take on cars like this; particularly since they realise that very often they are driven by mature drivers who will take very good care of them. However, you will probably need a specialist broker who can put you in touch with these insurers, unless you can find one through an enthusiast's car club.