In formal English, many readers object to is when, is where, and reason…is because constructions on either grammatical or logical grounds. Grammatically, the verb is (as well as are, was, and were) should be followed by a noun that renames the subject or by an adjective that describes the subject, not by an adverb clause beginning with when, where, or because. (See B2-b and B3-e.) Logically, the words when, where, and because suggest relations of time, place, and cause—relations that do not always make sense with is, are, was, or were.

Where refers to places. Anorexia nervosa is a disorder, not a place.

The writer might have changed because to that (The reason the experiment failed is that conditions in the lab were not sterile), but the preceding revision is more concise.