You also have to remember for which clubs Messi and Cristiano played in the Champions League. Not saying that it takes away from them, but if you add that consistency to other top players of the times, they'd also be on the spotlight a lot more often.
With Cristiano for example, he was scoring at regular levels until 2011 for someone playing for CL contenders for 5 seasons. Then football became this shit that we have today and all top teams got boosts while everything else pretty much died. Everyone had inflated stats and stars from Real Madrid and Barcelona in particular had hyperinflated stats, which is a testament of their superior quality. But it's like 'icing on the cake' difference rather than actual difference making most of the times.
If you compare both Messi and Cristiano scoring in knockout stage from their start until 2011 and from 2011 until today, you'll notice the difference. Okay, for Messi that's reduced as his club for the past 4-5 seasons has been underwhelming when compared to the earlier part of the decade.
Zlatan played for great teams but when this thing happened he was stuck at PSG with a squad that wasn't that great altogether, something like a Moratti fantasy with very good attacking players but a team that lacked balance or identty and tried to relied on individuals too much.
And Zlatan had 6 goals in the advanced stages of CL during this time, missing several vital games as well which cost PSG the qualification to the next stage. It's not like Zlatan magically improved his CL knockout game stats (he was unlucky actually, especially playing for us), it's more that the level had changed and he enjoyed luxuries that were not there before.