Soap features in a number of curious English idioms, most of which seem to have something to do with salesmanship. To stand on a soapbox, for example, implies that you're selling something: it conjures up images of a shifty guy hawking patent medicines in the street, or maybe giving a political speech or preaching some religious message, while standing on an old crate that used to hold soap. (Why soap in particular? I'll get to that shortly.) To "soft soap" a person is to flatter and sweet-talk them, the way a salesman might. Soap operas get that nickname because they're firmly associated with daytime television and radio aimed specifically at women homemakers, who would be tuned into to broadcasts while doing laundry perhaps, and thus a good potential market for advertising soap products.
I've a fanciful hypothesis for why such idioms have tended to cluster around soap in particular, and not some other thing. To return to the street preacher on their soap box, they could be standing on an apple-crate, so why soap? My hypothesis is something like this: soap is one of those commercial products that benefits the most from being oversold. Most soaps do basically the same thing. There's different detergents and additives you can put into soap formulations; it's possible to make a soap that lathers more effectively or has some other actual benefit, but it's far easier for a cheap but clever capitalist to make soap out of the same standard-issue stuff that's good enough (castile soap, say, spiked with a little menthol or limonene to make it "smell clean") and count on pure marketing hype to make an average-quality soap seem like God's gift to washing-up. Soap is trivially easy to sell on purely emotional and even spiritual considerations, as if having the whitest linens and the shiniest dishes were hallmarks of divine perfection. And soap is slippery stuff, like sales talk. It seems only fitting that soap should have become a sort of running joke and symbol of insincere marketing, especially when coupled with romantic melodramas on radio and TV.
What's the end result? You go to the grocery store, and you see a hundred different soap brands, and do you really have any way to tell them apart beyond the price? Maybe you've some specific objection to soaps made out of certain materials, and you choose on that basis, but for the most part you've only got catchy brand names and highly decorated boxes and unprovable claims and boldly asserted percentages (of the "up to 47% more effective than" variety.) This is the world we're used to living in, saturated by advertising which gives us the illusion of a much greater variety in our purchasing choices than actually exists. How much practical difference exists between (say) all the different brands of liquid laundry detergent that you could buy? Trying to find out through empirical means, running experiments on all the different detergents to see which is actually most effective in practice, would be a dreary and immensely time-consuming project, so that's left up to Consumer Reports and other consumer-product testing organizations. It's unclear to me whether many consumers actually make purchasing decisions based on such studies. My guess is that most people tend to buy whatever's cheapest and if it's mediocre soap, they use more of it. And meanwhile the marketing boys still work overtime on sales pitches for soap; the secret to joy and bliss still lies in spotless dishes, at least for advertising purposes.
Possibly, Western society is still reverberating from the abrupt social changes that came from the mass-production of basic consumer products made to a reliable and acceptable level of quality. Before the advent of standardized mass-produced soaps and industrial detergents, soaps would have been made on a smaller scale from whatever fats or oils were available, and no doubt the quality and supply would have been variable to a degree that contemporary society simply doesn't accept any longer, except (paradoxically) as a luxury. In the past, a truly effective variety of soap might have seemed a precious rarity, perhaps something that you could get from only a few places in the world. Now soap is boring, it's a routine product that most folks buy without thought, assuming that it'll do its humdrum dependable job. Hence it seems ridiculous when marketing tries to pretend that a soap brand is something special, maybe even something that can transform your whole life.
I'm somewhat reminded of Jell-O, which once could be sold as a special treat because, before the mass-production of gelatin, making clarified gelatin was an immensely tedious process and therefore implied that you had a well-stocked and well-staffed kitchen. Now, you can get the stuff in a packet and it will always work, and thus Jell-O has become something of a joke in modern society. We are haunted still by the time when clear gelatin desserts really were something rare and special. If King Arthur had been served sliced peaches in orange-flavored gelatin, she would undoubtedly have felt honored with a truly royal dessert, one that only the finest kitchens could produce. The enthusiastic marketing for Jell-O tried to keep that feeling alive; Jell-O was still a kingly dessert, if you believed an old Jell-O advertisement.
Uh, kinda ran out of thoughts there. I thought I was going somewhere with that but
~Chara