Nonprofit Vote points out the obvious:

“If you want to rig a local election, there’s an easier way than stuffing a ballot box, gerrymandering a district, or amassing a campaign war chest to scare off challengers. Have your city or county adopt winner-take-all “At-Large” voting, where, instead of using districts, all or most council or school committee candidates must run “at large,” city or countywide.“

How does this make elections less competitive?

“At-Large voting could be seen as workable in the small-town scale of the 19th century. Today, beyond the inherent bias of At-Large block voting, the challenge of running across an entire city or county is compounded by the following:

• The high cost of running—staff, communications, mail (still needed), database management, and media;
• The burden of fund-raising to run citywide;
• The need to contact voters and gain name recognition in a jurisdiction that can often be larger than a state senate or even congressional district; and
• Added time away from work and family to campaign and raise funds.”

Read more here.