Because they are clearly trying to push posters into voting for Rantanen. That is why they posted playoff stats but not any regular season ones (which are a huge set of data to conveniently leave out).
Rantanen 281GP 91G 151A 250P 0.89PPG
Marner 300GP 83G 208A 291P 0.97PPG
These stats are with Marner averaging about 45 seconds less TOI/GP, and full minute less of PPTOI/GP.
I tend to agree with your point on the OP pushing the voting. But there's a bit of context needed in the numbers. Rantanen played 9 games back in 15-16 season when Marner did not even though they're from the same draft. I think it's fair to at the very least put emphasis on those 9 games. Removing them puts Rantanen's PPG to 0.92. Which is Still behind Marner's but it's quite a lot closer.
If we look at their last 3 regular seasons in a row, we get stat lines of:
Rantanen: 197GP 79G 133A 212P 1.08PPG
Marner: 223GP 64G 166A 230P 1.03PPG
Now, I'm not saying using their entire career totals in this comparison is inherently flawed, but I do think that comparing 2-3 previous seasons for such young players does give out a bit more accurate representation of them as players.
Note: I'm fine either way, I just thought your point merited a counter point.