Domain changed to archive.palanq.win . Feb 14-25 still awaits import.
[11 / 1 / ?]

Pokemon Loicences

No.44223222 View ViewReplyOriginalReport
When you start the game, you only got a right to carry two Pokemon. You're not yet a proven trainer and all of these bugs and birds you won't use anyway land in the PC.
This lasts until the first Gym Leader. For every new Badge, you can carry an additional Pokemon. You hit a full party of six at the 4th gym, exactly in the middle of the game.

It explains why everyone isn't carrying six Pokemon and it follows the natural progression of your team. It can also be used to set up imbalanced fights with your rival, who managed to get a badge before you did.