When it comes to tanks, Roadhog has always been a solid choice for players that want to play a more aggressive hero that can soak up damage. Despite how fun he is to play, he was conspicuously absent from most pro games during the first few weeks of Overwatch's launch.
The damage nerfs to McCree and Widowmaker in mid-June's balance patch mitigated some of the hardest counters to Roadhog's playstyle, and now we're starting to see him in more pro match team comps. To understand this shift it's important to know why one might pick Roadhog in a pro game, and how McCree and Widowmaker so thoroughly shut him down prior to the patch.
Roadhog puts out an impressive amount of damage for a tank, and he excels at punishing enemies that end up separated from their team. In pro games, he's most often seen when the objective is a static point. This is because he's even more dangerous when the enemy team has to come within his gun's range to attack or defend the objective.
On the flipside, Roadhog's biggest weakness is his lack of a shielding ability, which makes him unique among the tanks. Every tank has some form of damage mitigation, but every other tank's ability is some form of damage prevention shield. If you play as Roadhog, your damage mitigation is based on a self heal with a low cooldown - great for recovering from skirmishes, but only useful if you have the chance to disengage and use it.
Because of this, burst damage has always been Roadhog's biggest problem, and until recently McCree and Widowmaker were some of the biggest burst damage producers in the game. Either class could produce a flurry of powerful bodyshots that would stop Roadhog in his tracks before he got a chance to heal. To make matters worse, this burst damage could be done safely from medium to long range.
McCree's flashbang-fan-roll-fan combo could take out any Roadhog that didn't flat-out kill him in the first second or so after hooking him in. Roadhog is also the only tank without any way of avoiding a flashbang thrown at him. This created a sort of sudden death showdown between the two heroes in 1v1 - will McCree get his flasbang out before Roadhog's hook, or vice versa?
The biggest balance problem was that, if both stuns missed, McCree was still free to do a fan-roll-fan combo from outside Roadhog's weapon range without Roadhog able to do much of anything about it. Blizzard decided that was straying a little too far from his intended role as a slayer of Tracers and the like, so now that combo has been nerfed to the point where it can't outright kill a tank. This has made all the difference for Roadhog and his self heal!
Since June's balance patch, McCree has mostly fallen out of favor in pro matches. It turns out Roadhog can fill the anti-agility hero role pretty well with his hook and shotgun combo, too - Roadhog’s hook works as well as McCree’s flashbang for stunning hapless enemies, and his self heal gives him a way to stay in the fight longer than the gunslinger can.
It's true that Widowmaker's damage from headshots haven't decreased, so theoretically these changes won't affect the best snipers too much - but there's a reason why even the best pro CS players buy the AWP over the Scout when they can afford to. No one is going to get headshots 100% of every game, especially when trying to snipe quickly under fire. Widowmaker's nerf made the quick bodyshots all Widowmakers take from time to time less devastating. Roadhog's wide load of a hitbox is especially susceptible to these quick shots, and now they're weak enough that they won't ruin his day with a few shots from across the map.
Widowmaker has been picked less since the balance patch as well, though that mostly just means she's not a must-pick hero in every team comp anymore. Her area denial role has been taken over by Roadhog in many cases - while Widowmaker picks enemies off from afar as they approach a point, Roadhog stands on the point making trouble for attackers and pushing them away with Whole Hog.
If you're interested in seeing for yourself how Roadhog is played competitively nowadays, Gale Force's Harbleu has been regularly picking him to attack and defend points in.