Because unlike other sports, hockey's evolution in speed and skill has produced less space, better defenders and subsequently, lower stats, until recently that is. It's hard to measure astronomical totals from an era with such inflated totals against a modern era that doesn't put up remotely the same type of statistics while being just as, if not more talented.
The NBA and NFL evolved and are putting up higher offensive stats compartive to previous eras, the NHL is the opposite. This is mostly why, the other reason is the NHL fanbase is far more nostalgic than any other, they still think guys like Maurice Richard are top 10 players all time.
Ultimately, Crosby and Ovechkin are CONSENSUS top 10 all timers right now, it's just a matter of where. The comparisons in the NBA are based on the immaturity of the fanbase which likens to the Twitter era where everything is either the greatest or the worst. They have no sense of history in the game and I say this as a guy who grew up with and played college basketball.