With NASCAR ratings slipping in recent years (This year's Pocono 400 in June was the lowest rated race since a rained out Monday morning race in 2000), I'm sure that, aside from money, a big factor in this was the fact that NASCAR's best numbers to date came during their time with NBC. Maybe there's a hope at recreating that, as NBC did an exceptional job of promoting the series and generating interest during their first contract from 2001-2006. (NASCAR started controlling television rights in 1999. Before that, it was individual race tracks.) I'm sure they see more of an opportunity to grow themselves with NBC as opposed to the crowded ESPN, or with TNT who seemingly have given NASCAR broadcasts a B-level production when compared to their NBA coverage.
It should be interesting to note that, for the first time since the heyday of ESPN's SpeedWorld in the late 80s-mid 90s, NASCAR, Formula One, IndyCar, and ARCA (A smaller stock car series once relatively prominent, now struggling to field 20 cars each week.) will all be on the same network. It could be beneficial for all series involved if NBC leans toward a route of unity between them.
It's also interesting that 6 Sprint Cup and 17 Nationwide races still remain up for grabs to any network that wants them. FOX is likely to get the 6 Cup races, but it doesn't seem they want the Nationwide Series, as it runs on Saturdays and could interfere in MLB coverage, either on network or FS1.