Because there was no talk of anything: not even a jot on a fag packet as to what form this Brexit would take and what relationship we would have with our largest trading block where 50% of UK exports go. The opposite could be said: that the question itself made a confirmatory vote obligatory because the original poll certainly didn't tell us what people really wanted nor outlined any options.
The Norwegian model for example takes us out the EU and thus fulfils the Referendum brief as it stands. So whats the problem ? The single market?... Yet the referendum never asked anything at all about the single market to justify leaving it and here we are being steamrollered into leaving it on the grounds of what? The understanding that only leaving the single market and thus the principle of the free movement of people gets rid of "them foreigners". That's the only justification for the hard Brexit we are headed for: its nothing to do with nobly fulfilling the Brexit vote but appeasing anti-foreigner xenophobia.