"Gotta catch 'em all" has been the slogan for the Pokemon franchise.
Thus, when the original Japanese Generation II was released, they introduced 100 new Pokemons to catch.
The Pokedex also includes the Generation I Pokemons, which gives a total of 251 unique Pokemons.
On top of that, they introduced Shiny variants for the Pokemon, which appears extremely rarely and is a prized item for collectors.
For the less extreme fans, they (though somewhat misguidedly) added Unown, a Pokemon shaped based on the English alphabet with 26 different forms (in that generation) to collect.
To accommodate for players who wishes to fill their PC with all unique Pokemons (excluding Shiny Pokemons) and Unown forms, they also increased the PC Pokemon storage capacity from 240 to 280, just barely enough to fit them.
Correction
Um actually ... The initial Japanese release of Generation II doesn't have 280 space for the PC.
It only have 270, which makes it shy of the 251 + 25 (other forms of Unown) = 276 slots required to fill the PC with unique Pokemons and their forms.
For the international release, they increased it to 280.
Kind of hints that Unown was not well thought out.
Note: the 2 other forms of Unown, based on the question mark and exclamation mark, are introduced in Generation III.