When you ask this question I think there's a bunch of things you're not taking into consideration. Sure, some technologies were lost, then rediscovered, however you need to take the historic, technological, and cultural setting in which they were used into consideration.
Your common medieval peasant is extremely illiterate. In fact, so are most lords and nobles, with many not being able to do much more than sign their name, and requiring the services of some kind of scribe to write letters for them, etc.
This would be especially applicable in the times of King Arthur.
Furthermore, you have to realize that these people often lived hand to mouth. Disease would claim them - or their children - often. Famines were a very real threat. Plagues as well. Raiders? War? Oh, yes.
They lived in a world full of dangers and insecurity. Life was a heck of a lot more difficult for them - which really serves to mess with your priorities when you have to choose between putting food on the table or taking a bath.
And so, you've got a whole bunch of illiterate people scraping a living in the dirt. Only very, very rich people can afford big houses, or castles, because it's very difficult to maintain the things.
You build them because you want people to recognize your status, or because you need to defend yourself. However, keeping them in a state of repair is indecently expensive. Heating them, in particular, is a pain in the behind! Someone needs to cut the wood, stockpile it, keep it dry, etc. It's a lot of hard work, and that's expensive. There's only so many trees nearby, and everyone needs wood to survive the winter. Additionally, they're also the easiert material for people to build their houses out of, many of their tools, etc. There's a lot of competition for lumber. Running a fire in every room of your castle is an expense even lords could not afford.
And so, your American travels to the past, and meets King Arthur. He proposes a whole lot of incredible upgrades to his medieval castle. Brilliant!
Now then, how does one manufacture porcelain in the 6th century A.D.? More importantly, even if you've got the ability, how expensive is it?
Now that we're discussing porcelain thrones, we need piping to go with it. Unless your yank is an accomplished plumber you're out of luck anyway, as few people understand how the whole system works off the top of their minds. However, let's assume that our hero does know.
Well, you need metal to make those pipes. You need blacksmiths, and other craftsmen. However, these people are also needed to fix the tools and weapons your people require to survive on a day to day basis. Are you going to take them away from those vital responsibilities in order to build you plumbing? You could, but it'll cost you (for example when your enemy invades and your weapons/defenses are in a state of disrepair).
So now you've got this super fancy, expensive as heck, toilet installed. Congratulations, you may now do your business in style. However, was it worth it? Is this system feasible to be implemented in more than one glorious case? Probably not, because your average farmer has much bigger problems than that.
The problem with implementing many other large scale changes is the same: priorities.
The Romans had sewer system, complex baths, etc., however they were a very large and powerful civilization. They could afford those luxuries. Even then, you can imagine that not just anyone had a large, heated bath with water being pumped in and out of it, like some have been discovered. No, my friend, only the super-rich had those!
The reason we later embraced all these inventions is because it became economically viable.
For example, when we got better at farming, and our techniques allowed us to manufacture more food, we could spare more people to advance our societies in other ways.
When our cities grew, and the lack of hygiene spawned disease after disease, we once again figured out why Romans implemented a sewage system.
Sometimes these technologies simply need to wait for their time to come.