It is a great myth that the '94 MLB strike and the cancelled World Series had a long term impact on MLB - and that it didn't recover untill the magic Mark and Sammy show of '98. It's a great story, unfortunately it ain't true.
1993 and 1994 were abberations. The addition of the expansion Marlins and especially the Rockies (who played '93 and '94 in the much larger Mile High before moving into Coors Field in '95) provided a non realistic bump in attendance. By '96 MLB was back at it's pre expanion level and by '97 it had significantly exceeded it - all before the Mark and Sammy show. MLB attendance has stayed remarkably steady since then - topping 30K/game last year.
MLB 1990 2,105 54,823,768 26,045
MLB 1991 2,104 56,813,760 27,003
MLB 1992 2,106 55,870,466 26,529
MLB 1993 2,269 70,257,938 30,964
MLB 1994 1,600 50,010,016 31,256
MLB 1995 2,017 50,469,236 25,022
MLB 1996 2,267 60,097,381 26,510
MLB 1997 2,266 63,168,689 27,877
MLB 1998 2,430 70,601,147 29,054
MLB 1999 2,428 70,139,380 28,888
MLB 2000 2,429 71,358,907 29,378
MLB 2001 2,429 72,581,101 29,881
MLB 2002 2,397 67,390,074 28,114
MLB 2003 2,411 67,630,489 28,051
MLB 2004 2,400 72,968,953 30,404