As the answers here indicate Ubuntu .iso s are not expected to boot if copied with dd to a USB thumb drive.

Now my question is why is so that some Linux distributions have the option to directly write their bootable .iso file to a thumb drive with dd but some (read Ubuntu) have not(for Ubuntu I think it has to be converted to .img first). Is it for some architectural difference in .isos? Or is it due to any limitation of dd itself?

I don't know if it is off-topic here. I can move it to a more proper place if the community thinks so or suggests one. Some explanation would be appreciable.