Here's why:
* Skill Drain's Effect: This card negates the effects of all monsters on the field.
* Debris Dragon's Effects: Debris Dragon has two effects:
* Summoning Effect: Special Summon itself from the hand if you control a Level 4 or lower monster.
* Continuous Effect: Gains ATK equal to the number of "Debris" monsters in the GY x 500.
Since Skill Drain negates all monster effects, Debris Dragon's summoning effect and continuous effect are both suppressed. It becomes a simple monster with its base ATK and DEF, unable to use its special summoning condition or gain additional ATK from the graveyard.
In short, Debris Dragon is effectively useless under the influence of Skill Drain.