I believe the "100 years" statement refers that the theory is general, but requires deep understanding and new results in representation theory and algebraic geometry to progress, something that might be slow to progress (I want to make a comparison to number theory, but I am not sure how apt it is).
Also, there is a loss of precision when translating to the algebro-geometric world: Instead of proving a lower bound against the properties of a complexity class (i.e. polynomials that vanish when objects in that class are given as input), you are proving it against its Zariski closure (of aforementioned polynomials). It is conceivable that in order to separate the two, one has to examine the boundary of that closure (those polynomials that occur only in the closure but on the original set). It is believed that in the determinant vs permanent variant of the GCT program, this is likely the case.
Finally from personal experience, the skillset required to understand GCT in depth is quite different than what is usually the focus of undergraduate or even master programs in CS, essentially picking up the prerequisites is a natural follow-up of choosing to study GCT.