9. Out of the great respect for Scarlett O'Hara shared by all transvestites (and those who have played transvestites), he decided to make his suit out of curtains.
(Actually, maybe we should vote on whether the suit was more sofa- like or more curtain-like).
8. The network wouldn't let him wear the (compromise) #1 fan choice: eyeglasses and Red Speedos.
7. (This assumes the suit was grey, as it looked on my, admittedly poor-reception, TV): It was a code for X-philes: in the season premiere Mulder will discover he is really a little grey man.
6. In a foul attempt to make our hero look bad, the Mitch Pileggi (apologies if I butchered the spelling) Estrogen Brigade substituted the ACK suit for DD's real suit at the last minute.
5. The patterns on the suit contain hidden encoded messages. I could not decipher them myself, but Frohike thinks they contained GA's email address, Langley thinks they spell out the whereabouts of Samantha Mulder, and Byers says they give the location of a store which really DOES have copies of the 'I want to believe' poster.
4. That wasn't DD at all, it was the mighty morphin Schwarzenalien.
3. Ratboy threatened to expose DD's secret alien love child unless he wore the suit.
2. Not fully secure in his own sex appeal, DD felt the need to wear a suit which would be sure to make fans want to rip it off immediately.
And number 1, (predictably, alas): the reason is an X-file which will not be solved until the series finale.