It is a feature of the game that prevents you from researching along a single tech path for too long without researching in a different area. Each time you select research, an average of 1/3 of the techs are "hidden" (exept if you are playing a scenario).
There is a file in the game called the "rules" file with a list of techs. Each tech is assigned a number, 0, 1, or 2. You will always be able to research the first available tech in the list (the first one for which you have all prerequisites), otherwise, however, you will be unable to research any tech associated with the remainder of your current tech number (all techs you have researched, excluding starting techs, but including the one you are just about to start) and 3.
This is actually a rather important point of strategy, because researching techs in the correct order and trading at the right time can allow you to get the techs you want sooner.
Check out the stories and tales forum for some succession games; there is a fair amount of planning discussed there, and tech order occures fairly frequently, especially in the early game. The thread "The People's Republic" has a rather decent amount of information on the subject, if I recall correctly.
