According to the following [source][1], it is not clear why gender agreement (which existed in OE) disappeared. It was probably due to simplify communication between Old English and Old Norse: 


> -  If you speak another language like Spanish or German, you are familiar with grammatical gender. In Romance languages (and many others), nouns have a gender. In French, a chair is la chaise, a feminine noun, and a hat is le chapeau, a masculine noun. But did you know that English used to have gendered nouns too? 

> - ***Until the 1200s, English had grammatical gender. Instead of using the articles “the” or “a”, Old English had a masculine article “se” and a feminine article “seo”***. The sun, for instance, was feminine, so it would be written “sēo sunne”. If you referred to the sun, you would even say “she”.

> - ***However, in northern England in the 1100s, grammatical gender disappeared. Historical linguists aren’t entirely sure why this happened, but Professor [Anne Curzan][2] suggests that genders were lost because of the language mixing that went on in Northern England during that time***. 

> - Between the 700s and the 1000s, there were Vikings invading northern England where peasants lived. The two groups spoke different languages: Old English and Old Norse. However, it is quite likely that many people were bilingual and fluent in both languages. ***Both Old English and Old Norse had gender, but sometimes their genders contradicted each other. In order to simplify communication, gendered nouns simply disappeared***.

(blog.dictionary.com)

According to the following extract from [Quora][3], the introduction, and usage of the gender-neutral article "þe (the)" is probably responsible for the decline in usage of gender agreement in the English language. ***I personally think this is an interesting assumption*** so I add it as additional information on this topic:  

> - Unlike most other Indo-European languages which have gender, English introduced an article — þe (the) — that was gender neutral. Since most of England in the 13th-17th centuries was uneducated, it was a great time for language change. The article þe was much easier to learn than arbitrary gendered articles, so it started to be used by everyone.

> - And around the 14th century, Colloquial English started to drop the declensions present in proper Old English, and with it, grammatical gender disappeared.


  [1]: http://blog.dictionary.com/oldenglishgender/
  [2]: https://en.wikipedia.org/wiki/Anne_Curzan
  [3]: https://www.quora.com/Why-dont-nouns-in-English-have-gender