Why won’t the remain parties work together for the EU elections?

These elections should see the pro-Europeans triumph. While Labour still prevaricates, the Liberal Democrats, Greens and Change UK (the Independent Group as was) are the only unequivocal singers of the EU ode to joy. They all campaign for proportional representation, yet seem to have failed in that spirit of cooperation and coalition. If, just for this one election, they combined as a single pro-EU, pro-referendum grouping, they would do far better. Not only would they win more seats and votes, they would shake Labour off the fence for fear of being eaten alive.