There is a good explanation on reddit of the history of this. [0] The Core developers did agree to a block size increase though in what is commonly referred to as the Hong Kong Agreement. They didn't follow though on that agreement and made up some truly weaselly excuses for why.

More recently a large group of players in the space came together and agreed to what has come to be called the New York Agreement (NYA) This said that they would support Segwit and a 2MB hard fork for bigger blocks. Core and Blockstream who largely controls Core have ignored the NYA and worked against it. They are pretending like segwit has support w/o the 2MB block increase when it doesn't and they are going around doing some truly cringe worthy stuff to tray and scuttle the 2MB part of it.

[0] https://www.reddit.com/r/btc/comments/6uw5kf/why_does_core_r...