Roy is my secondary, I main DK now (used to main Samus)
The underlying reason is that I play low tiers only, my religion doesn't let me play high/top tiers.
I think the true reason I play Roy is that I feel he is actually a "balanced" Marth. His move speed, lag, attack speed, fall speed are fair for someone that has a sword with large disjointed hitboxes (heavy sword, wears armor). To me Marth is a very broken character; he is the same as Roy but for some magical reason he is floaty (despite also wearing armor, swinging a sword and being the same weight), his aerial attacks are faster AND stronger than Roy's even though his sword has less mass. I think it's pretty unfair to give a character such disjointed hitboxes; insane grab range and then a bunch of other bonuses/advantages on top of that, then put him in a game with characters like Mario, Pikachu, Yoshi, etc.
The only real problem with Roy (and I'm sure the devs screwed this up big time) is that his aerials do so much less damage, less knockback/hitstun AND they are also slower than Marth's. It still pisses me off thinking about it. Speed for Power. Simple game design 101. In an attempt to re-invent Marth and re-balance a new char they basically Pichufied him.
Also I'm 99% sure Marth is a lesbian, based on the hairdo and matching hairband.