Because Model S is already so feature poor nobody thought things could get any worse? Expecting evolution from Model 3, that of course would also trickle to Model S/X, is pretty normal given the state of the company making the car. Tesla getting some things ready in time for Model 3 would have made perfect sense.
Let's not kid ourselves, beyond the unique sigature features (big battery, big screen, autonomous with updates), Tesla's feature-sets aren't normal for their current class (S/X), nor are they normal even for the upcoming class of the 3 if the current appearances are at all accurate. They are far below of what normally is expected. Have you seen the toys an Audi A4 gets these days? Of course you pay for the toys, but they are available.
Surely people still buy Teslas for those unique signature features, but it is not a stretch to have believed Tesla would have improved e.g. the interiors through the Model X and Model 3 design processes. Alas, it seems not yet.