Why are there no ||= or &&= operators in C#?

maybe just use

isAdmin = isAdmin || IsGroupAdmin()

I guess it is partially because a ||= b is kind of confusing because there might be two versions of the implementation: a = a || b, or a = b || a. And they act differently because the right-hand side of the expression is sometimes not evaluated.

Why did the logical operators get left out? Is there a good technical reason why it is hard?

They didn't. You can do &= or |= or ^= if you want.

bool b1 = false;
bool b2 = true;
b1 |= b2; // means b1 = b1 | b2

The || and && operators do not have a compound form because frankly, they're a bit silly. Under what circumstances would you want to say

b1 ||= b2;
b1 &&= b2;

such that the right hand side is not evaluated if the left hand side does not change? It seems like only a few people would actually use this feature, so why put it in?

For more information about the compound operators, see my serious article here:
https://docs.microsoft.com/en-us/archive/blogs/ericlippert/compound-assignment-part-one

and the follow-up April-Fools article here:
https://docs.microsoft.com/en-us/archive/blogs/ericlippert/compound-assignment-part-two