Good question! The potatoes get even smaller if you believe the LAX shooter and the bus slasher were also Islamist terror. The best answer I can come up with is that it is the nature of the distributed organization: not everyone can blow up a city, or topple the WTC, or blow up a ship, or shoot a bunch of random people, etc., nor is the money source consistent, so their operatives do what is within their capabilities and their budgets.
On the one hand, you are right: Why wait to pop a nuke? On the other hand, if you can discern a pattern to why bomb here or there on a large scale or small scale, let me know - I don't see it.