It's surely much easier to stop the referendum from happening than to deal with it if the result goes against. Obviously if there's only a turnout of 2% it'll be fine - but it looks bad enough stopping it from happening - let alone suppressing a result you've lost.
As I understand it, the Catalan assembly doesn't have the power to call this referendum - and it may even take a change to the constitution for the central government to allow one. But much better to face it early than wait - as support for independence looks to be growing. Pro-independence parties got a majority in the regional elections (for the first time ever I think), as I understand it on the promise of this referendum rather than because a majority want independence.