A lot depends on what you mean by "SMS marketing". People love timely, helpful messages - such as reminders about the deadline to get in their views on the planning application for the end of their street or an update on the venue for a meeting of a local organisation they are part of. Good campaingers have access to plenty of information people would like; they do have to be good campaigners though :)
The latest Ofcom survey shows that 94% of UK households use mobile phones.
Now consider what proportion of UK households have a usable letterbox for delivery of campaign leaflets; i.e. exclude those rural homes without a letterbox, those urban blocks of flats with just a door to push leaflets under, the multiple occupancy houses with a communal hallway but no personal letterboxes and so on.
And then there’s that property on the electoral register which, despite you circling the block four times, taking a peak from the skies through a Google Map satellite view, stalking the postie one morning and even consulting the Land Registry you still cannot find.
Certainly in my experience that all adds up to more than 6%.
That puts in an interesting light the relative effort most campaigns make in getting to people via their mobile phone compared to getting to them via their letterboxes.