Those chemical weapons are cheaper, easier to obtain (literally a grocery store), easier to produce, easier to obtain higher yields, etc. They also are extremely terrifying.

But even chemical weapons aren't used that often. We rarely hear about anthrax letters and we never hear about terrorist attacks that used bleach and ammonium even though practically everyone knows about this reaction and the materials are cheap, easy to obtain, and don't raise suspicion. (Side note: we do hear about people accidentally creating this mixture fairly frequently. Enough that almost everyone knows someone that did it)

Chemical/biological/nuclear weapons are just not worth it to terrorists. If they were we'd have seen them and if we're being honest the dirty bomb is the hardest out of all of them.