Part of the reason is that it's simply easier for a "Do Brexit NOW" party to corner the market than a "stop Brexit and also a tonne of other things" party, which somehow has to argue that the Greens, LibDems, etc, are also part of the rotten system. And I don't think they could sell that very well.
Change UK would, in my opinion, have been better off staying out of the EU elections altogether, and focusing on Westminster politics for the time being. I also think they should have stuck to being called TIG, I'd like to have seen them working with, rather than against, the LibDems, and ... well, a whole bunch of other things differently. I still think that there's a case to be made for a new centrist party, made up of the non-extreme Labour/Tory components, and it may even be needed more than ever in the short term, but I'm somewhat saddened to see this group waste that opportunity.