It is like asking why America commemorates 9/11. Just like the downfall of the World Trade Center, the attacks and killings happened at Victoria Terminus, Hotel Taj, and other places in Mumbai on 26/11/2008 had left the wounds and horror that are hard to heal, and impossible to forget. For the world, it may just be the mourning on one day, but for those who lost their relatives, and had the first-hand experience of the tragic mishappening, it’s a fear that has taken a permanent place in some corner of their houses and hearts, perhaps replacing those whom they lost on 26th November 2008.
So answering your question, it’s not a ceremony that needs to be observed, but it’s a habit which will not stop haunting us even if we want to get rid of it.
The second part of your question says why don’t we move on and fight terrorism. Let me, first of all, make it very clear that the fight against terrorism has nothing to do with the traumatic memories of the day. In fact, the fighting becomes easier and more effective when we are able to keep the cause in mind and to fight that cause. If the attacks of 26/11 crippled households, they also strengthened people individually, and taught them how to survive in the most atrocious times.
Steps have been taken, not only by India but also by America to uproot terrorism as early as possible, but the authorities can’t go to the extremes on which terrorists function. As they say, “an eye for an eye makes the whole world blind”.
So if you indulge in the senseless arguing like why Ajmal Kasab’s verdict took 4 years while he didn’t wait for even a microsecond to take thousands of lives, the simple counter argument for this would be –that’s the difference between a state and a terrorist. War against terrorism is itself a war and no state can afford to bargain the lives of innocent people for avenging the wrong done. Not even in the name of God.