>The Pokémon is only available on the second visit to the mountain, after revisiting Buck's grandfather's house in the Survival Area.
First you have to do that, and on order to go to the Survival area you have to complete the Sinnoh Pokedex.
>Survival Area (Japanese: サバイバルエリア Survival Area) is an area in northern Sinnoh accessible after completing the Sinnoh Pokédex.
Sources: Stark mtn. Bulba, Survival area Bulba