The simple answer is because they didn't exist yet. The origins of the Commander (or Elder Dragon Highlander) rule set date back to 1996, as discussed in this forum post. They've been modified many times by various people since then.
So the real question is, why didn't the official Commander rules get modified to include Planeswalkers when they were introduced? The most likely answer seems to be that many Planeswalkers would not be well balanced for Commander. Searching the internet will reveal a whole lot of complaining when WoTC started allowing any Planeswalkers to be used in Commander, and Mark Rosewater discusses in this article about introducing them that
Ethan knew there were two major hurdles the team had to overcome. One,
they had to prove that they could make designs that would work.
Planeswalkers are not always a perfect fit for the Commander format,
so the design was going to be tricky.
So it seems that rather than allow them, but then go through banning a bunch that didn't work, they were just disallowed completely, and exceptions like the recent ones are just written on the card itself.