Disposable ones are single use and, er, disposable... using them more than once is leaving you open to ear infections etc 
Why are non disposable ones less likely ? Same ear I suppose. But other than that...

I hate the bloody things personally. Doesn't matter what type, from head phones to in ear plugs and buds they make the ear sweaty and itchy eventually. Head phones less so obviously but they all irritate in the end, and with a helmet on you can't adjust them without stopping, gloves off, helmet off , dick about

Unless your on the motorway for more than 10mins I would not bother personally. I good fitting helmet that plugs that channel between the neck and jaw leading up the the ear is much more effective in limiting noise ime, when taking into account convenience and fafing about putting reusable ear plug in your ear then finding the little clean box to store them in. Pita.
However if your spending long periods at motorway speeds then they are a must really. Although they restrict hearing engine rpm which I find awkward balancing blipped downshifts to road speed, clutch bite point, etc if there's other noisy road users about.
I guess the ideal design would be noise cancelling above a certain dB. But no idea if such a thing exists. Bose have only just come out with noise cancelling music in ear buds so I doubt it, and would be mega bucks anyway. Plus there's still a sizeable lump outside the ear that probably wouldn't work with a helmet on.
I did consider the in ear moulded ones but they would have to have speakers in them for music as well to get full use and justify costs and all eventualities for intercoms and sat navs etc.