Fear Uncertainty and Doubt.
The remainers are playing up the scaremongering to extreme hype levels because they don't have any real facts that are totally down to the EU doing something good. Many of what they spout could just as much happened if the UK was outside the EU. Look at all the other countries in the world, they aren't part of the EU, but have they suffered? Sure some have (mostly socialist despot ridden countries, a bit like the EU :-) ), but overall the world is a better place than when the EU was created, and it isn't down to the creation of the EU.