First cost. Caesium is relatively expensive and the hydroxide is somewhere in the region of $1 per kg. There arepanies that make caesium stearate and other soaps but nobody buys them for use as a soap. I am unsure of the price but you can rest assured that it will be far more than sodium based soaps. You make a soap by reacting a base with a fatty acid then separating out the soap from the glycerol. If this is not easy you have to first make the fatty acid and react it with a base. I have no idea how hard it is with caesium soaps but I suppose it does not matter that much if you leave it in. However major soap makers want glycerol. There is ament about toxicity below. That would mainly relate to ingestion so as long as the soap stays out of your body that is probably not of great concern. However another issue is what the properties of the soap would be. Sodium soaps also have a rather pleasant ability to form good cakes and they are not particularly alkaline so they give a pleasant feel in their use. However cost and availability far outweighs everything else. Sodium hydroxide is a major chemical product and is available in the hundred thousand tonne range. Caesium hydroxide price would skyrocket if a major volume use arose so quite simply manufacturers use the cheapest and best alkali available.