Why wasn't Isaac put in Melee to promote Golden Sun?

There's always a chance that Sakurai didn't have the time to add in a Golden Sun character without having to delay Melee itself. It really depends on when that Golden Sun game was first announced(which looks like a year before its actual release) and then how far in development Melee was already. They could of already had the Melee roster decided by then and either the creator of Golden Sun was late in asking for Isaac to be added as the game was to far in development(I believe that was the case with Snake since his creator wanted to put him in Melee).

I'm also comparing this to the fact that they added Fire Emblem characters to promote them and other countries didn't see a Fire Emblem release untill 2003.
Golden Sun came out a few months after Melee, but that doesn't mean that they shouldn't promote it since it would be a new upcoming franchise that would become popular in it's own rights like Earthbound is.
Anyway it should of been Isaac instead of Pichu since I just dont know why they put Pichu in if they have Pikachu anyway.

If memory serves, wasn't Golden Sun originally a gamble?

They had no idea it would be as popular as it became. A new Fire Emblem game was sure to make money and thus safe to advertise.

Ok then lets compair Isaac to Lucas shall we?
Lucas was going to be in Melee instead of Ness, but Nintendo decided not to do it because Ness is more iconic than Lucas and Lucas was the main character of what, a cancelled game.
So here's the thing, in Melee there was no Lucas, no Isaac, but there was a Pichu, and in Brawl what happened they put Lucas in as easy addition to the roster, and all that Isaac got was a stupid NPC spot as a assist trophy.
This isn't right I know Isaac is way more popular and iconic than Lucas will ever be, so how did this happen?

He better be playable!
I thought him being an assist trophy was an insult, Golden Sun is THE BEST RPG EVER even better than final fantasy and I will have Isaac playable in Brawl.

the reason for pichu in melee was that they simply needed a fourth pokemon character and they wanted to make it fast so they needed a clone.(not to mention that pichu is like the mascot of the 2nd generation making it the lucario of that time).

As for lucas:
It probably helped,that he was planned for melee and since he is a semiclone he was easier to include than isaac(but it is still weird,they didn't include isaac)

All that matters is SSB4, whats done is done (In this case, Melee)