I'll start my answer with an overview of the history of English and conclude with a short paragraph on the reasons for language change.

 

History of English: The history of English starts with Old English and the arrival of Germanic tribes (the Angles, Saxons, Jutes and Frisians) around the year 450, who brought their Germanic dialects with them. These Germanic tribes called the native Celtic tribes wealas (meaning "foreigners", it's the origin of the name Welsh). The vocabulary of Old Englisch was mainly of West Germanic origin with some Scandinavian influence.
Old English was an inflected language, similar to other languages such as Modern German or Latin. Inflected language means that the grammatical function of words was mostly determined by inflections. Similar to Modern German, Old English nouns used inflection to not only express number, but also case and gender (masculine, feminine and neuter; the grammatical gender did not necessarily have to match with the biological sex).
Similar to nouns, adjectives and pronouns had very many different cases.

 

Following the 1066 invasion of Britain by William the Conqueror, the phase of Middle English (~1150 - 1500) started. The invasion massively changed society and politics in Britain, with French being established as the language of the nobility and the upper class. While English was still widely used by the masses, French had a huge influence on English with many French words still being present in Modern English.
During the period of Middle English, a widespread loss of inflection took place, causing English so shift from an inflectional to an analytical language. This caused the word order of English to be more fixed and reduced the amount of inflectional endlings in nouns and adjectives, thus losing the grammatical gender.

 

Early Modern English (~1500 - 1700) is usually connected to the introduction of the printing press in England starting in 1476. The increased circulation in printed books increased the standarisation of English and lead to the introduction of norms of spelling and pronunciation.
This period saw a huge influx of the vocabulary of other languages, mainly Latin, Greek, Spanish and French.
Early Modern English grammar was already very similar to Modern English grammar, with a fixed word order (Subject-Verb-Object), as well as the same inflectional system.
The Great Vowel Shift was a phenomenon that started during the Middle English period and lasted for about 400 years. During this shift, all the long vowels of English were either raised or diphtongised, which is why Modern English sounds so different from Old and Middle English [(there was a great answer on this by bloodswan in a similar thread)] (https://www.reddit.com/r/AskHistorians/comments/6q37mx/when_i_hear_chaucer_spoken_in_middle_english_it/dkuj8bw/?st=j5qsevm5&sh=2da4b13e).

 

Around 1700, the period of Modern English started. Though there is no distinct historical event, the date is usually quoted because English had mostly reached its present state by then.
With the colonisation of large parts of the world, English started to spread to various countries and cultures, becoming the standart language in many modern countries (the US, Canada, Australia, New Zealand).

 

There are very many differerent resons why certain aspects of languages change over time. Often it is hard to determine a single reason for language change, with factors such as geography, social prestige, external influx, or even imperfect learning playing a role.

 

Sources:
Bieswanger, Markus, and Annette Becker. 2008. Introduction to English Linguistics. Konstanz: UTB.
Crystal, David. 1998. The Cambridge Encyclopedia of Language. Cambridge: Cambridge University Press.