Well if he had zero money he couldn't retire to a quiet country life... That's why homeless people today do not choose to live in nice places in the countryside but instead sleep on the pavement in cities. It's much less pleasant but you starve less in the city.
Also, maybe he wanted to tell other people they were fools, not just "wanting to be seen not giving a fuck".
Sure, if you want to lecture people about themselves then you give a fuck about at least that, but it's different than simply trying to be the center of attention.
I really think Diogenes was a philosopher (Diogenes the character -- this is true whether he actually existed or not): he had a point of view that he wanted to share.