kmhome,
skill are accessible in two ways.
1st is when you reach lvl 15 - then you select your specialization. since every specialization has different skills it makes no logic to allow you to randomly select any skill (skills are balanced per hero type).
2nd, you cannot max all new skills, some skills require you to reach a certain lvl (20/30/40...), this is because some skills if maxed in early stages of the game, can extremely benefit with compare to other (for example, having 5 mercs in lvl15, will make it too easy, so it is limited atm to 3 in early stages).
its all about balancing the heroes and the game difficulties, along different tiers.
As you will move through the different tiers, you (as did I), will learn and understand more and more on how it is all balanced.