The main reason that we provide 5 different elastomers is that it allows us to have a lot of distinct stiffness settings (15 different possible combinations using a single elastomer or a combination of 2). The key to this is that when springs are used in parallel (which is how the elastomers are setup in the stem) they act as if they were a single spring where the spring rates are summed together. This works in practice because of how rigid the stem is - when a load is applied to the handlebar it is transferred to both elastomers and the combination acts against the force as if it was one combined elastomer. This means any combination of different elastomers will sum to one stiffer elastomer and allow for lots of potential stiffness settings.
So, why don't we provide 10 elastomers with two of each durometer? The main reason is that it is not needed - the additional elastomers would allow for more combinations but the jump in stiffness between each stiffness setting is not large enough to require any other intermediate setups (i.e. it is really hard to tell the difference between 60/90, 80/80, and 70/90). The chart below shows all of the elastomer combinations in order of increasing stiffness. The numbers correspond to durometer which is a Log scale meaning each jump in 10 units is actually 10 times more stiff (e.g. a 90 is 10 times stiffer than an 80) so the larger number always dominates and the smaller determines incremental changes.