Here's why they did it:
On Tuesday, officials released a statement saying that the government of Ecuador “respects the principle of non-intervention in the internal affairs of other states” and had cut off the internet access available to Assange because “in recent weeks, WikiLeaks has published a wealth of documents, impacting on the US election campaign”.
The statement also reaffirmed the asylum granted to Assange and reiterated its intention “to safeguard his life and physical integrity until he reaches a safe place”.
https://www.theguardian.com/media/2016/oct/19/wikileaks-ecuador-julian-a...
They also said in the letter that the action does not prevent Wikileaks from carrying out its journalistic (sic) activities.