Because as one of the few relatively healthy financial states ( U.K.) .the ECB would open a very large financial vein in an attempt to keep the Italian corpse warm if it crashes. Or were you expecting Bulgaria and Luxembourg to shoulder the load?
During the last financial crisis we were, I believe, under no obligation to bale out any Eurozone country.
Where we did so (such as Ireland) it was because we recognised the impact financial failure on our doorstep would have on the UK.
Brexit changes none of this.