I can think of two reasons to not use a pot or other drug named coin.
1. Maybe you live in an area it is illegal to posses. You get busted for some possession, then they see your stash of weedcoins. Bam, additional charges might get filed here. Maybe a paraphernalia charge or a dealing charge.
2. How many people want the government to have a record of every pot related purchase you make with one of these cannabis coins? I think people want a little more privacy.
People used bitcoin on the first silk road because they thought it was anonymous. The last thing they wanted was a coin with a name like cannabis coin that tells the whole wold what you are using it for.