Yes terrorists want to terrorize, but a group of terrorists is not like an invading army that can take and hold territory. They are extremely limited as to what they can do due to resources, personnel, and logistics, so they need to ensure that whatever they do yields the maximum return possible. They are also very driven by symbology, as was alluded to in rr's comment. The fact is that Islamic fundamentalists do not view Las Vegas as being any more decadent than any other American city. We view it that way because most of us don't live in cities where prostitution and gambling are so prevalent, but attributing that same sentiment to Islamic Fundamentalists is just silly.
Any terrorist leader has to pick a target that is going to yield a maximum return on the message they are wanting to spread, will get the most press resulting from casualties and or damage to infrastructure, and has a great chance of success... otherwise the mission is not worth risking and wasting resources on. So with that in mind, why would a terrorist ever target Las Vegas? They have a relatively low population density, there's no strategic or logistic value to the place, there's no major media, just sand, hookers, and gamblers. So... do you plan out a mission and expend resources to blow up a hotel and kill a couple of hundred people, or do you plan a mission where you can kill thousands or tens of thousands? Where you can cause major disruptions in business, transportation, or finances? Sorry, but planning an attack on Las Vegas would be as silly as planning an attack on Mechanicsville.
If another attack occurs it will be in Los Angeles, Washington DC, New York, Chicago, or another major mainstream city... not Vegas.