But imagine the mischief one could have with all that ammonia and a tanker of iodine crystals. ;)
Which explains why so many radial miles are evacuated following rail-line disasters in which ammonia cars have a risk of cracking open.
Hydrogen sulfide, hydrogen cyanide, or a large number of other gasses are much more deadly.
Those agents are more tightly controlled than ammonia, thus making them a less-than-favorable choice in the eyes of al Qaeda architects.
As history has demonstrated time and again, terrorists do not go normally for the "most deadly" agents in their attacks. They are not looking for optimal death yields; they are looking for optimal terror in the survivors (since the dead no longer have anyhing to fear). Thus, they prefer a good number of those attacked live in a maimed capacity (whether physically and/or psychologically).
In the mind of the terrorist, high body counts are desirable only if the survivors are frightened enough that they will pressure their governments to cave in to the demands of the terrorists.
An LPG tanker ship would be one hell of a bang.
Big bangs are low on terrorist priority lists. Human psychological impact is a much higher priority. Note that the 9/11 attacks were not powerful enough to destroy the Pentagon (at all) or the WTC (initially). Indeed, the WTC destruction only succeeded because the twin towers' tube design left all the load-bearing on the fatally-compromised outer shell. Had the buildings been standard design, it probably wouldn't have collapsed as soon.