Because EU treaty is quite specific. It's not possible to expel a member. A member may choose to invoke Article 50 to start a process to leave at a timing of their choice. The UK has had a referendum which Parliament must now consider whether to offer up a bill giving the PM of the day authority to invoke Article 50, which in turn must gain royal ascent. This process is quite clear and EU politicians/diplomats opinion on the timeframe is frankly irrelevant.

Of course there is negotiating benefit for the UK to delay invoking Article 50; and it's no surprise the UK will delay the process given the difficulty the UK will face once the 2 year count down starts. The EU has the right to refuse negotiation prior to Article 50 - and that's the game of chicken we now find ourselves in.