Why does 'dd' not work for creating bootable USB?

You are writing the image to the partition 1 of /dev/sdb

Change this to the following command:

dd if=/mint/iso/image of=/dev/sdb oflag=direct

this information was acquired from here

You copied the image to the first partition. Try copying to /dev/sdb rather than /dev/sdb1.

The actual mechanism varies a bit depending on the type of image you're using, but for simple DOS/MBR images you need to get a correct partition table (with the bootable partition marked as being bootable, and the MBR - the part of the initial 512 bytes that isn't the partition table - containing initial boot code.