Why does try..finally block not register the original exception as suppressed?

Because try-with-resources is syntactic sugar and the Java compiler doesn’t expand regular try-finally blocks in the same way.

Take a look at the following code:

try(FileInputStream fstream = new FileInputStream("test")) {
    fstream.read();
}

When compiled and then decompiled (using IntelliJ IDEA) it looks like this:

FileInputStream fstream = new FileInputStream("test");
Throwable var2 = null;

try {
    fstream.read();
} catch (Throwable var19) {
    var2 = var19;
    throw var19;
} finally {
    if(fstream != null) {
        if(var2 != null) {
            try {
                fstream.close();
            } catch (Throwable var17) {
                var2.addSuppressed(var17);
            }
        } else {
            fstream.close();
        }
    }
}

Whereas this code:

FileInputStream fstream = new FileInputStream("test");
try {
    fstream.read();
} finally {
    fstream.close();
}

Looks exactly the same when compiled and decompiled.

Now, the case could definitely be made that all finally blocks should be expanded in the same way as is done above, but for some reason that has either been overlooked or decided against.

I suggest you open a feature request for this because I think it’s a sensible feature.