I guess because assisted suicide is still illegal in many countries. I guess other products used for suicide like SN, inert gas, rope- are more of a grey area because they do have 'legitimate' non lethal purposes. Still- you wouldn't really buy a suicide pod for any other use- meaning the retailer sold it to you- and likely provided instructions KNOWING what you were going to use it for. It could convincingly be argued that they assisted a suicide.
As to why assisted suicide is still illegal- religious, economic, emotional reasons. Plus- if it WERE to be legalised- I imagine the majority of people would want it to be heavily regulated. I doubt we'll be seeing the Sarco Pod on Ebay/Amazon any time soon...