The spint cup series is the top tier level of nascar racing in the USA. Think of it like the premier league. Even includes relegation of sorts where by the top 33 cars in points are guaranteed a position in the next race unless you're a former champion whereby you can never be exempt. So with another 10 positions available, you can sometimes get up to 30 competing for those 10 spots 'usually the big paying races, other wise around 15'.
The nationwide is the feeder series where alot of those 20 cars and alot more fight it out. Alot of the bigger teams like Hendricks and Rousch have development drivers in this series, and there is nothing stopping every driver competing in the series, its just that its a level below and generally not as difficult. It adds an extra 500 miles + of racing to each weekend so it doesn't tempt the big guns like Jeff Gordon, Jimmie Johnson etc.
Also most of the time you see the big guys competing in this is to give their team 'manufacturers points' so that they can earn more money to pay for the racing. It's more cost effective then the top tier with slightly slower cars.