Why are only certain system maps available in the galaxy map?
Solution 1:
When you start in Elite, your ship's nav computer comes pre-loaded with the system data for some systems. I can't find an official statement for this, but the community consensus seems to be most systems with over 1 million in population are pre-loaded (there are exceptions).
For the rest of the systems, you have to visit yourself to get the system map (and other data).