"The loss and weakening of unstressed syllables at the ends of words..... had disastrous effects on the inflectional system, since many endings now became identical." — (Barber, 1993: 157)
There is no simple answer to the question, why exactly English has lost the majority of its inflections. Here's just one idea:
- The articulatory stress began to fell on the first syllable;
- The (various) inflection endings began to be uniformly reduced to schwa;
- The inflections have been subsequently unused, obsolete, and abandoned;
- The inflection markers have been replaced by more complex syntax features of the language;
At the end of the Old English period (end of the 11th century), the word endings (containing inflectional markers) became less articulated:
- Inflection vowels such as -a, -e, -u, and -an appeared to be uniformly reduced (weakened) to -e, (pronounced
[ə], or schwa).
- Word-final -n after -e apparently lost in unstressed syllables. With the course of time, the remaining -e was abandoned as well.
For example, Middle English drinken (from OE drincan) became first of all drinke and then drink (ref. Baugh and Cable, 1993; Burrows and Turville-Petre, 1992).
The same source claims that the example word drink got to its final stage in the North by the 13th century, but spread to other regions by the 15th century.
It is said that the same effect has occurred to in most dialects of Old English.
David Crystal (ref. 1995:32) suggests a possible explanation why such reduction may have occurred. Through the evolution of the Germanic languages, in most words the articulatory stress fell on the first syllable. He suggested that such stress pattern may introduced some difficulties to the audibility of the inflectional endings, especially in day-by-day conversations, especially with phonetically close -en, -on, and -an.
One should also consider one of the potential implications of Chomskyan linguistics, suggesting that all human languages are equally complex. This means, in particular, that a language with a simpler morphology would more likely to have a more complicated and non-straightforward syntax. This implies that OE has gradually developed a more complex syntactic structure to replace the obsolete inflectional endings.
The principle of constant complexity is widely disputed, however.
Other theories well may worth your further research, including:
Leith (1996) draws attention to the argument that the OE inflectional system was inefficient. For example, In the case of mann (man) and hand (hand) there is little distinction between the cases:
Sing Plural | Sing Plural
Nominative mann menn | hand handa
Accusative mann menn | hand handa
Genitive mannes manna | handa handa
Dative menn mannum | handa handum
...hence, the entire inflectional system may become abandoned due to its incomplete usefulness.
Others observe that OE could significantly reduce its inflections because it became a trade language after the Anglo-Saxons defeated a Norse invasion in 878, leading to its simplification — as suggested in "Adventure of English" by Melvin Bragg, 2004;
As the other answers suggest, pidgin and creole languages often appear to be simpler than "local" languages. David Crystal (see above) considered this factor important;