Pac called Nas the ringleader for very logical reasons:
1) Nas was the first non Wu member to appear on a Wu album when he appeared on "Verbal Intercourse"
2) Nas was also on Kool G. Rap's album
3) Nas was being heralded as rap's messiah
4) Nas appeared on Mobb Deep's Infamous album
5) Nas appeared on Dre's Aftermath compilation on a song called "East/West Killas"
6) Nas drops It Was Written and it features several artists that Pac was dissing including Mobb Deep, Lauryn Hill, and Dr. Dre (who is on the intro to "Nas Is Coming" throwing shots). There's also the introduction of The Firm, which Dre was kind of a part of.
^^^ knowing all this, Nas looks more of a ring leader than Biggie. Nas has enough clout to pull in a beat from Dre at the height of the East vs. West tension. Not to mention, he also has beats from Stretch. Nas looked more of a threat than Biggie. I'm not saying Nas purposely was like, "I'm gonna put all these pieces together and go at Pac". It just happened that way, and to someone who really looked at it as war, how could he not think Nas wasn't the ringleader?