>>55500545The new Pokemon are your main incentive to progress.
If every new Pokemon was available on Route 29 how many people would actually bother doing anything other than the bare minimum to get through the game?
How many people would even turn the game back on after the credits to find out you can go to Kanto?
Chances are not really that many.
This was back before Pokemon had anything for you to do other than catching, battling and going to the gyms and such was merely a means to an end - even in RBY with its pitiful post-game locked a whole other area with tentative new Pokemon behind becoming champion, you weren't just battling for the sake of battling.
And even now Pokemon still employs this design ethos to a certain extent - it's no longer the sole focus due to battling having become much more complex and multifaceted with more rewarding feedback loops thanks to battle items and such, but you're still gonna be encountering new Pokemon (be they new to the series in general or merely new to the specific game) until the very last areas of the game.
