Here's why:
* Stat-boosting moves are typically focused on a single stat. Some moves raise multiple stats, but usually by only 1 stage each.
* There are moves that raise defense and special defense by 1 stage each. Examples include:
* Cosmic Power: Raises the user's Defense and Special Defense by 1 stage each.
* Bulk Up: Raises the user's Attack and Defense by 1 stage each.
While there's no single move to achieve this, you can achieve a similar effect by using two moves in a row, such as using Cosmic Power followed by another move like Amnesia which raises Special Defense by 2 stages.