In the past several decades or more, there has been a tendency in American English to omit the article where it previously was obligatory in phraseologisms. Thus, for instance, one could hear today the phrase in the wake of uttered by a reporter on the NPR program “Morning Edition” without the article, i. e., “in wake of . . .” Other examples of this phenomenon are “on par with” and “on air” (instead of the traditional on a par with and on the air). By the bye, the absence of an article in the phrase in hospital differentiates British usage from American, where in the hospital is steadfastly maintained despite the tendency to drop the article elsewhere.
There already exist many examples of phraseologisms that omit the article (like in cahoots with, in defense of, etc.), so it is not surprising to see that the trend is toward omission of the article rather than toward its insertion.
A possible explanation resides in the nature of a phraseologism, as contrasted with a simple prepositional phrase containing an article. In the first case, a unified meaning of the words as parts of a whole is paramount, rendering the individuating function of the article as a grammatical category subordinate to the syntactic unity of the phraseologism. But in the second case––that of a simple prepositional phrase–– the article retains its role as an individuator.
This ongoing change toward phraseologization as it affects articles is an instantiation of the principle of language structure whereby the implementation of the rules of grammar always tends over time toward the iconic realization of the ontological definition of the categories involved.