My argument in a previous post was that such lack of preparedness was both surprising and unwarranted. Whether it can be blamed on isolationism, I don't know, but if anyone has an opinion, I would be happy to read it.
Hmm, well, we'll probably never know, but at a wild guess, it may be just a matter of priorities.
While for the UK and Japan shipping was essential, by virtue of having a moat around the country

, shipping along the coast of the USA seams somewhat less important. The USA had a pretty good road and railway network to get just about anything from point A to point B, so even complete interdiction of maritime trading along the coast (which never actually got THAT far), would not be as crippling.
I'm also guessing that the sheer lack of ships made it necessary to prioritize. The USA had given most of its destroyers to the UK, and while they weren't very well suited for sub hunting, they were still better than nothing. This left the USA rather short on destroyers, and with even fewer in the production pipeline. So, you know, between coastal shipping, trans-atlantic convoys, and the whole war in the Pacific, SOMETHING had to draw the shorter straw. As I was saying in the previous paragraph, shipping along the coast seems like the most obvious choice for leaving unprotected.
Then there's the issue that the destroyers they still did have, were piss-poor designed for sub hunting anyway. They were designed as fleet screening ships, which means among other things that high speed counts, turning rate not so much. For sub hunting it's the other way around. Anyway, since you're comparing it to the brits, the UK had good ASW destroyers, the USA pretty much didn't. Even when they did start sticking destroyers in the convoys, it would be some time until they started actually making all that much of a difference.
I'm also guessing that it was a matter of guessing wrong. The Type VII submarine, which comprised almost ALL the German sub fleet, just didn't have enough fuel to go all the way to the USA and back even on the surface. (Except maybe if you put a mast and a sail on it

)
Only the very new Type IX had enough fuel for that, and even that one had to make most of the way on the surface. There's a reason why Germany sent only 5 subs there: because it had 6 of those new subs total, at the time. The sixth was having mechanical problems.
Since those were new, I'm guessing the USA might have thought that it's simply outside the range of German submarines at first. Which means they also hadn't planned to build good ASW escort ships either.