At first I didn't buy the McDavid personality thing, but as I think about it, it does make sense. If you are introverted, even keeled and perhaps even somewhat expressionless, it will be hard for the media to grab a soundbite out of you. Without media complicity in marketing, an athlete's prominence and exposure is an uphill climb.
Would McDavid be well known if he put his foot in his mouth once in a while, said stupid things, maybe say politically incorrect things? Maybe - but even then it depends on what media outlets are willing to cover him. During the hockey season, hockey has to compete with basketball, which tends to have more colourful (personality) characters, as well as off-season news in other sports. Media outlets also realize that the Canadian sports landscape is not as homogenous as it once was, and they area almost obligated to cover other sports just for variety's sake and to keep the interest of a diverse viewership base. For me if I see any soccer news, I just turn the channel but for many, they tune in to soccer news.
Perhaps hockey has a reputation as a pretty sportsmanlike (for the most part compared to other sports) sport where athletes go in front of cameras in interviews wearing suits and such. These traditions may not have the marketing drawing potential as "bad boy" or "bad girl" athletes and sports where the participants can mouth off about this or that or do this or that and get the gossip news coverage, all over it. Look at the amount of NFL players (and their families) who seem to get coverage for their views on social and political issues, even though I don't know if anyone actually cares what they think. When was the last time an NHL player talked about what was going on in the world. That's just not how the hockey climate is, and that's a good thing, but it also reduces the press coverage.