In first first division sports leagues, they can afford to pay their players more and buy better players than their lower division counterparts.
Since I follow soccer, I will give you details on them.
MLS run their league differently from the NASL and other lower divisions. MLS have clubs that are franchise's. That mean the club ownerships and player contracts are all owned by the league. Unlike other soccer leagues, they do not have a full free agency where players can pick their own teams. Only recently this month, player fought to havevsome form of free agency, but got a bullcrap version in the end were players 28 years of age with 10 years of playing in MLS can have free agency. MLS's is the only soccer league in the world the puts a major middle finger to the true form of soccer. It's not competitive because the teams are not allowed to get consistently stronger than they're weaker counterparts in the same league.
The NASL follows traditional soccer. Free agency, independent club owners and a plan for promotion and relegation is the main thing that sets both MLS and NASL apart. MLS is a self gain league and they have a lot of fans that love them for it. The NASL is less selfish and plans on taking risks MLS is to afraid to do. The NASL is also a very new league that just started playing in 2011. MLS fans hate the NASL because the NASL's plans are to replace MLS as the new first division. NASL are known as big dreamers with big and some time unrealistic plans. The unrealistic plans are more realistic than MLS's, if I may add.
The new NASL have no connection with the old. They only share the league name with some clubs using old NASL club names like the New York Cosmos. I'm actually rooting for them to reach their goals and somehow reach even greater.
The USL is the third and final professional league. Unlike the MLS and NASL, the players pay to play instead of getting paid. In 2013, the USL made a deal with MLS to let MLS reserve teams into they're club. Thanks to that, they are the largest professional league in soccer pyramid. Originally USL-PRO, they changed it to just USL with MLS ripoff jersey logo with a plan to become second division to replacing or being ranked with the NASL.