This is because the program is in c#.
I would say that the main reason you cannot use MQL indicators is because they do not include logical conditions like "Indicator rises" or "Indicator changes its direction"...
Both products - MT and FSB are very different. MT is designed to give the possibility to program your own expert. On the other hand, FSB gives you a User Interface and Generator for creating experts as fats as possible without programming.
An ordinary MT Expert uses the indicators to receive values and applies all logical rules over these values. The experts do not expect the indicators to provide logical rules.
However, the FSB Pro strategies provides to the users the rules included in the indicators code and the indicators returns calculated signals.
Every MQL indicator can be translated in C# with adding proper logical rules.