The Conservative Educational Forces...
The fact is that more than 95 percent of all university level writing assessment (not classroom eveluation, by university-wide assessment involving entrance requirements or promotion barriers) uses either a standardized MC test (SAT/ACT) or a one-time, timed, impromptu writing sample, usually graded holistically. While there have always been exceptions (as the 5% indicates) there has never really been a strong push for assessment alternatives. Even the call now for more portfolio based assessment and on-line portfolios is a mere drop in the bucket.
Assessment is a political thing because students are judged on their writing ability and problems with writing is often--mistakenly--equated with a lack of intelligence. So schools (especially public ones) need to prove that their students can write. So far, the only cost-practical means of dong this is through standardized testing.
Its important to note that at the university level, we don't have standardized testing to check on students' knowledge of history or algebra. I mean there are other standardized tests (MCATS/GREs) and some departments in some schools have qualifying exams for their majors, but outside the classroom, non-instructor assessment is by and large a thing we invoke to test just our students' writing ability. As a writing instructor, this fact insults me--as if my judgment of my students' abilities should not be trusted.