Simple: the first exists, the second doesn't. ;-)
I was doing a crossword today and was stuck for a word - the clue was 'humble'. In the end, with a little help, it hit me - 'lowly'.
We talk of a lowly birth, dwelling, background or even person, but not standards. Standards are just 'low' or 'high', or even 'exacting'.
So lowly means humble, poor, squalid and miserable. We don't use it a lot, more in prose than conversation.