Roger Olson is spot on. I would also add to his reasons the two often get confused is that it is easier for some folks simply to write off orthodoxy as another form of fundamentalism, so they don't have to take it seriously.
So what is the difference between "orthodoxy" and "fundamentalism?"
Orthodoxy is belief in the universal doctrines (dogmas) of Christianity rooted in Scripture and commonly held and taught by all the church fathers and Reformers. They what author Gary Tyra (in Toward a Missional Orthodoxy) calls the "Christological verities." They include the deity and humanity of Jesus Christ (incarnation of God), Trinity, salvation through Christ and his cross, and salvation by grace alone.
Fundamentalism is (among other things): adding secondary and even tertiary beliefs to basic Christian orthodoxy as NECESSARY for authentic Christian identity (e.g., premillennialism, biblical inerrancy, young earth creationism), insisting that salvation depends on belief in a long list of doctrines including ones NOT PART OF basic Christian orthodoxy, and refusing Christian fellowship with other Christians who are "doctrinally polluted" or "doctrinally impure" because they do not believe everything on the fundamentalists' long list of essential doctrines.
Anyone should be able to see the difference between Christian orthodoxy and fundamentalism.
The entire post can be read here.