According to the following [source][1] it is not clear why gender agreement (which existed in OE) disappeared. It was probaly 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? (We were recently inspired to write about grammatical gender because the hosts over at Lexicon Valley, Mike Volo and Bob Garfield, discussed why languages have gender on three excellent episodes.)

> - ***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)


  [1]: http://blog.dictionary.com/oldenglishgender/
  [2]: https://en.wikipedia.org/wiki/Anne_Curzan