It's just hard to find one Eevee in Pearl and Diamond. Why is it?
Eevee is a special Pokemon in the Trophy Garden, which means it has to be one of the last two Pokemon mentioned by the owner, Mr. Backlot. He mentions a new Pokemon every 24 hours and you can soft reset which Pokemon he mentions, but you have to do it in the first place, otherwise Eevee won't be there at all.
This also requires the National Pokedex, otherwise only the "normal" Pokemon appear in the Trophy Garden.
Finally, when Eevee does appear, it's still only a 5% chance, so you'll have to search for a while.