The "Save Our Sixpence" campaign was just one factor, and probably the least of it. London Underground's plea probably had most to do with it. Just think of the role the Tube plays in the country's economy, getting all those Londoners to work every day, not to mention the importance of "the City". And parking meters for cars throughout the country - yes, the car contributed to the economy - important to keep motorists happy.
If you could fool the housewives that THEIR plea was the one that had been instrumental - or allow them to fool themselves - well, that was an excellently devious ploy for a politician.

I became interested in coins again in 1979, after hearing that the sixpence would be demonetised. Late that year, I got some from a bank, and decided that the poor beasts should enjoy a little freedom before being melted. I bought something for 10p in the student union shop and proffered four sixpences to the lady behind the counter. She looked late 50s, maybe early 60s. She looked at the coins and groaned, but with a smile on her face. "THOSE horrible things!" she said. "You've been saving them up - haven't you?!" Just to show you how quickly sentiments change. The coin and its design (or designs) just looked out of place among the decimal system. The 5p could have been reduced earlier, had the 6d not occupied the approximate slot needed. And no, they didn't turn up in change often at all. Shopkeepers didn't like them, and they quickly became unpopular with the public, once their value was a messy 2½p and not a nice half shilling.