because he stayed on the trump campaign longer then manafort, was on the transition, inauguration committee, etc.
all of this while being a known pro-russian lobbyist like manafort.
(read this profile on gates:
https://www.thedailybeast.com/despite-russia-controversy-paul-manaforts-partner-is-still-lurking-around-the-white-house)

so even after manafort left...gates still maintained his relationship with the trump team and could have been an embed/handoff for pro-russian interests.

manafort is the most recognizable name.
but gates could have greater exposure/liability (and more to tell).

as far as manafort goes...they were up against the clock with the statute of limitations and wanted to pin down these charges. finanacial crimes are commonly the most easily traceable/proveable (al capone). mueller intentionally assembled a team that had vast experience with money laundering and corruption. he has a gameplan and this is *exactly* where he wanted to start.

so the story isnt how these charges are unrelated to the campaign (though many in the media will sensationalize it as such because that is the only goalpost trump has them focused on). the real story is how we are only 5 months into the mueller investigation and he has already proven that he is operating with methodical precision.