Just looked at their squad:
3 Salvadorian internationals, 1 Kenyan international and 1 Venezuelan international. Javi Marquez has played over 150 matches in La Liga for Espanyol, Mallorca, Elche and Granada. Some may remember Brazilian born Amauri who was one-time capped by Italy and was best known for his time at Juventus.
Not happy he dropped down to NASL, but he does look to be surrounded by a few decent players.
They probably offered him more than he'd get in MLS. When he left for Japan he was making about $300,000 a season and he probably didn't take a pay cut to play in J1.
We don't know what happened exactly at Shimizu, but I saw them play Gamba Osaka (one of the top teams in Japan) last November in the Emperor's Cup. He was next door in Korea with the national team that day, but I remember watching their centre backs from the stands and thinking "there must be more to Jakovic being frozen out than just football". Like I said, we don't know what happened, but with Shimizu getting relegated, perhaps he was on a high salary and they were trying to force him out?
At any rate, Cosmos have the highest wage bill in the league don't they? He is probably getting double what he was likely being offered in MLS (if there was an actual offer).