There are a number of reasons for this.  Let us explore a few of them.

• Incumbent parties generally lose ground in midterm elections.  There is almost always some degree of backlash against the party that is in power as the Democrats were in the early '90s.
• Clinton's relatively liberal early agenda.  By the '94 elections, Clinton had pushed for universal health care and for allowing gays in the military.  These turned off many people who had voted for Clinton for economic reasons.
• History.  By this point, the Democrats had controlled the House for almost 40 straight years and there was some sense that Americans were getting tired of this.
• The Contract With America.  Drawing on these last two points, the House Republicans, led by Newt Gingrich, set out a list of promises they said they would keep if given a majority.  These promises played on Americans' dissatisfaction with liberalism and big government.