English might be the most analytic language in the IE family, in that it has no case, no gender, and very few personal pronouns. Since PIE and other IE languages are generally synthetic, then what drove English to be so analytic compared to others??
The quoted paper below seems to ascribe this to the time when the Anglo-Saxons were conquered by the French. Because Anglo-Saxons were the conquered people of the time, they found it necessary to communicate with the French and this profoundly affected English, not only in its vocabulary, but also in its grammar. The article summarizes the analytical development of English and its lack of inflectional grammar having resulted from this time period.
Language changes tend to stem from the want or need to become more regularized or simplified. For example, contact between two distinct yet similar languages produces a basic need to communicate for trading and other common purposes. The inflectional endings, in these particular interactions, become superfluous to the task at hand. Rather than attempt to learn the respective language’s unique inflectional system, two speakers of different languages can instead opt to learn the foreign word absent of its appropriate inflectional morphology.
German in contrast also had a great deal of influence from French as well, but was NOT the result of conquest and didn't have the profound affect on its grammar as it did in English.
French influence on the German language and its people, however, occurred not as a result of conquest, but rather admiration. Waterman (1966) notes that even before the Middle High German period, “the prestige of French learning and culture had… been firmly established in Germany” (p. 89). In fact, by the time of the Middle High German period, it was not at all uncommon for the German knights to visit in France, or even to seek service at one of the French courts. Nor was it unusual to find Frenchmen engaged as tutors to the children of German nobles. Thus, in a relatively brief space of time, the German language of the late twelfth and early thirteenth centuries took on many French words, expressions, and turns of speech. (p. 89)
Thus German still kept its largely inflectional language intact.
Innshort: Foreign speakers and rapid growth.
Grossly Simpmifying: The problem is that inheritence in linguistic terms implies an unbroken descend from parent to child. To say that the language changed, i.e. changed itself, implies an unnoticable, ever so slight difference from generation to next generation, at least in the usual explanation of sound-shift (I do suppose that hearing impairment due to infection must have been frequent, for one). Loss of morphemes can stem from sound shift, if sounds in various environments are lost. Reading of course slows the process, but only if a significant number of speakers can read.
Anyhow, there is no unbroken descend when foreign speakers adapt. Rather, they speak broken English to various success. The Norman invasion marks the start of Middle English. The rest is history as they say. In other words, to be honest: I don't know nearly half enough about the history, or the olde Languages; Just living in a city with 10+ % foreign speakers gives a good idea of the effect, though I have no idea under which conditions this could overcome the native speakers. Loss of irregularities is preferable, I suppose (viz. gender that has little to no semantics).
German on the other hand, since a comment mentioned, has a hint of slavic influence, which has even more cases (I'm generelizing from Russian, scold me if that's wrong), and vice versa (e.g.Grenze, nischtz [nichts]).