Here's why:
* Skill Drain only affects monsters that are currently on the field. It does not affect monsters in the hand or the deck.
* Summoning a monster from the hand to the field is a one-time effect. It does not involve a continuous effect that Skill Drain can negate.
Therefore, Skill Drain will not prevent a monster summoned from the hand from using its effects, even if it normally would be affected by Skill Drain.