The National Football League
The National Hockey League
The National Basketball Association
Major League Baseball
Major League Soccer
The "big" sports in this country all have their own top tier leagues. Nobody questions the food chain (domestically) in each league's respective sport at the professional level. Everybody acknowledges that the NFL is greater than the UFL, for example. Or that FIFA sanctions MLS higher than the USSF-Division 2 League. Heck, the other three have minor league systems feeding into them.
I suppose one could argue that the Arena Football League can lay a claim to being a top tier football league as well, since they play a different variety of football than the NFL. But most would still say that the AFL is little more than a minor league or proving ground for the NFL. After all, Kurt Warner came from the Barnstormers to the Rams and not the other way around. This type of movement indicates that the AFL is a development league slotted between the college game and the NFL.
So where does that leave lacrosse? There is both a professional field league and a professional box league. In addition to Major League Lacrosse and National Lacrosse League, a third professional circuit has come about. To be fair, the LXM Pro Tour does not tout itself as a league per se, but they do consider themselves to be the top destination for professional laxers and boast names like Kyle Harrison and Mikey Powell.
Lest we forget, a second professional box lacrosse league, Major Series Lacrosse, plays its games during the traditional Canadian season. MSL is the oldest professional lacrosse league in the world, but is not recognized as being on the same level as either of the US based leagues. Despite its history and significance, MSL comes across as a lower league than the NLL.