The House Impeachment Managers and Ken Starr went with Monica Lewinksy because of the dress. They had solid proof that Clinton had lied about this affair.
The House Managers and Ken Starr were also looking into Mena and Whitewater; they knew if they could find enough evidence on these two occurances, they would have a solid case for Impeachment. But they couldn't find enough evidence and went with just Lewinsky.