Why would government observers complain that the filibuster allowed a minority to control the Senate

Because from an outside prospective, a filibuster could virtually put the entire senate into a deadlock from a minority delaying or blocking a vote all together. This also means that in order to end a filibuster, you would require a "cloture vote" which is a super majority (67 out of the 100 senators) essentially taking power away from the majority.