A trend from the 1960's
Salvo and Vim and later Wisk tabs all failed for these reasons:
After a while consumers were realizing several things
a) they were having to handle the detergent with their hands
b) laundry for anyone who does enough of it soon realized that laundry does not come in one size load, nor one level of filthy.
c) unit dosing means you are constantly making a mental calculation to add up your laundry to match the dose instead of having the convenience of washing what you need to wash when you need to. Laundry already is a nasty chore to most people , try adding the chore of thinking about it all week long to match your 'pod' , 'tablet', 'unidose'!
d) when I visited ChesterMike 5 years ago I brought a few boxes of Persil pods back with me- I found out a disturbing thing about the "water soluble plastic" they used, it even came with a warning on the box!!
At times ( which was every wash by the way) the plastic might not dissolve all the way so be sure to seperate the remainder from the load BEFORE YOU PUT IT IN THE DRYER! It also stated never to use PODS in a COMBINATION WASHER-DRYER because of this. Yet more work, picking through wet laundry!
So I tried it by overlooking the instructions!!, I got a real messs in my dryers and combos at the end- the plastic had first melted then hardened either to the drum or some piece of clothing and had to be picked off by hand, yet more work for the "laundry-master®"
A trend for the lazy. In addition I will admit ; I was a big fan of Wisk Tabs because they cleaned if you used hot water but the uni-dosing was a PIA when I wanted to wash a pair of jeans and a top to go out with! But I think the point made above was true, to get you hooked they boosted the cleaning power on the new product to generate buzz about it, once it caught then back to the regular formula levels to increase profit margins.
Remember their costs are set by the chemical producers so it doesn't matter what they make, liquid, powder, tab, pod the base costs drive pricing on all product forms across the market. You will pay more for "convenience" and in the end I think consumers are very price conscious and convenient conscious .