But I think I might have an idea which explains, at least in part, why. Of course it's all conjecture. It's always conjecture and educated guesswork.
|Theseus vs. the Minotaur|
My personal theory is that this is one reason why his people wanted nothing more to do with him as King by the time he got back-- we see it echoed later in Byzantium MUCH later, where members of the royal family who they never wanted back near the throne again had their noses cut off, because if they were not whole they could not RULE-- and so, perhaps it was this same rejection which knocked him out of the running for that series of statues. Not just because he couldn't rule, but also because his very appearance was an affront to Athenian sensibilities about beauty. Theseus might very well have been better off dying with honor than coming back to Athens so physically disgraced.
|Theseus vs The Minotaur. Again.|
Add to this that most of the imagery we have of Theseus revolves around his youthful adventures: his trip to Crete to battle the Minotaur, his capture of the Marathon Bull, MORE of Theseus slaying the Minotaur (seriously it is EVERYWHERE), Theseus battling along the six entrances to the Underworld, and you wonder if he was admired as a King at all. Even in modern art and discussion, we gloss over his adventures as an adult. Both Mary Renault and Margaret George completely disregard his abduction of Helen, while Rick Riordan implies that he did not live to old age when Theseus' shade appears to Percy Jackson as a youth.
In fact the latter half of his life is just one long fall from grace so full of tragedy that it isn't really even all that heroic, completed with his death falling from a cliff. The only idea of Theseus as King we really have is the Sanctuary of Theseus in Athens, where slaves could flee to claim protection. This too, may not have made him a very popular King among the political elite, since it might have been construed as an upset to the status quo.
Now, of course there could have been other reasons, and in fact the website I linked to above states that it was simply a decision made by an oracle after a larger list was submitted. BUT, I don't think anyone researching Athens can forget the political nature of, well, everything they did. I humbly submit after laying all of this evidence before you, that perhaps Theseus's exclusion had a lot to do with the fact that Athens really preferred to forget about the fact he was ever their king, not ONLY because of his disfigurement, but certainly as the final nail in his coffin.
Ultimately, Athens had no use for a disgraced King, hero or not. Not when his father could do the job without the taint of dishonor and failure, certainly!