He's the obvious suspect, the only person of interest named by LE, and his actions and behaviour since Susan's disappearance has done nothing to reduce suspicion, but have in fact done the opposite.
His alibi is ludicrous, and he has done nothing to back it up. LE's investigations have not been able to establish any veracity to his alibi.
A couple of days after Susan's disappearance, he rented a car and went missing for hours, driving hundreds of miles, without telling LE what he planned to do, or on his return telling LE what he had been up to.
He has totally failed to cooperate with LE since one or two early interviews in the first few days since Susan's diappearance. He engaged an attorney (for free) who is known for defending guilty murderers.
He has not cooperated with friends or family in the search for Susan, or talked to friends or family about the event.
He has made no physical effort to find Susan that we know of, or expressed any regret at her disappearance apart from some on-camera snivelling that seemed more like regret that he was likely to spend the rest of his sad life behind bars, awaiting the death penalty.
The only individual attempt to help find Susan, the Susanpwell.org website, reads more like a Josh Powell advertisement, does not link to or credit any other efforts to find Susan, and even threatens legal action against anybody who uses anything from the site to help find Susan.
Facts that have been made public since Susan's disppearance, mostly about what a wife-abusing sad sack of sh1te Josh is, support the theory that Susan was likely to leave him soon if he did not pull his finger out and shape up, something he seems incapable of.
All just my opinion, of course.