You need players in the MLS

One thing that you'll notice from the top of the MLS aggregate table is that the top three clubs all have one important thing in common: An international superstar that headlines everything. In first, Bastian Schweinsteiger leads the way for Chicago. Toronto has Michael Bradley and Jozy Altidore while NYCFC leans on David Silva, among others.
While soccer is, in general, a fluid team game, it is apparent that having a premier talent, really at any level of the pitch, is intrinsic to the success of any club. The salary structure of the MLS almost necessitates such a move.
Most domestic players must fall within a low salary cap figure, however rosters are also populated with designated players, which don't need to be confined by those same strict rules, and allows American clubs to compete with big money international organizations. There is some level of roster building that allows for clubs to build from within, but the best players will always be compelled to follow the money, which is often found abroad. The differentiator, then, is the designated players.
As a result, the teams that are willing to spend the most on a handful of the best players are going to be the squads that have the best chance of success. Having one or two elite positions is going to elevate any squad to the top of the league, as has been the case in Chicago, Toronto and New York this year, but has been in the past with Los Angeles and Seattle. The right thing for any struggling thing to do is keep their ears to the ground to find a player that is ready to move to America after a long career in Europe, like Schweinsteiger, or to snap up a young player from South America ready to make their move on the international scene. Francisco Calvo might just be the best player for Minnesota United, and the Costa Rican is playing for the first time outside of Central America.
There will continue to be nuance in filling out the roster for most franchises, even those that have star players. Following the Compass team NYCFC has Silva, but has been graced by the addition of Alexander Ring this year, and will need to find a suitable replacement for injured Ronald Matarrita during this month's transfer window. Undoubtedly, the supporting cast is an important reason why they are near the top of MLS this season, but it is Silva that puts them over the top.