I always believed that the reason for that, was probably because it's the only one that had time to start developing a 'core'. It is the last Lavos Spawn that you have to fight, after all.
I don't suppose you could offer "a direct quote from Belthasar," which "would provide a more accurate answer to this dilemma," instead of "venturing wild guesses and speculating?"

Where's the rest of the level then? Why is it only this location in the game, that the party doesn't get to fully explore?
In stead of venturing wild gueses and speculating, a direct quote from Belthasar would provide a more accurate answer to this dilemma.
Actually, I never assumed
any "level" in the game is supposed to represent the entirely of the local. Consider Crono's house; there is only one bedroom, and only one bed, but two people live there. Are we to assume that Crono and his mother sleep in the same bed? Or indeed, as there are fewer beds in the game than there are NPCs, what does everyone do at night time? Are there shifts for who can sleep when? For those who don't get a bed, well it doubly sucks for them since they'll be going to sleep hungry, what with the total lack of farms in the game and all. Even for the ones who get a bed, things still don't look good for their future, as the world of Crono Trigger must suffer horrendously from inbreeding, with the world lacking a sufficient and diverse population.
So Lavos could be on Death Peak, just somewhere that wasn't included in the level, just like Crono's mother could still have her own room, or there could be farms in the world, or how towns might have populations in the triple didgets (rather than 20ish at best).
Right. That advice isn't any use at all... to you, having already played the game countless times and have pretty much memorized all the key aspects of climbing Death Mountain.
Actually, that advice isn't any use at all to anyone. If you'll allow me to be anecdotal for a moment; I volunteer in a nursery, watching over and taking care of kids that are 18-24 months old. One of the reasons children that age need to be watched is that they have a tendency to climb. If they see something they want that is out of reach, they have no problem pushing a toy over to it and climbing up it. If "pushing" and "climbing" are problem solving abilities that 2year old have, I'd expect anyone playing this game (and certainly the Crono Co.) to be able to figure this conundrum out without the help of the creations of the genius behind Project Kid (and really, it calls Belthasar's intelligence into question if he thought this was vital information that the team couldn't do without). As an aside, apes display this same sort of problem solving abilities.