Prior to 2005, suicide bombings were rare in Afghanistan, with perhaps one or two occurring since the war of liberation from the Taliban began in October of 2001. But during 2005 there were approximately 25 such attacks. And so far this year there have been eight such attacks.
This indicates a significant shift in Taliban strategy, perhaps reflecting an understanding that conventional attacks are becoming increasingly unprofitable. The Afghan troops, trained to Western standards, are proving quite lethal. The American troops are still around, and now the European NATO forces are moving into Taliban territory. So it appears that, for the Taliban, the only effective weapon they have left is the suicide bomber.
So far almost all of the known suicide attacks in Afghanistan have been against Afghan government officials or Afghan and Coalition security forces, rather than the much more indiscriminate attacks being perpetrated in Iraq by the al Qaeda faction there. This may be the result of influence by the Pakistan-based al Qaeda "center," which has realized that indiscriminate attacks, especially those that kill women and children, are a major reason why local tribes in many parts of Iraq are turning against the al Qaeda.
Also, suicide attacks are not part of the Afghan culture. In fact, despite their fearsome reputation as mighty warriors, Afghan combat tactics tend to be quite prudent. In battle, Afghans don't like to take chances, and go to great lengths to avoid taking casualties. Thus it should be no surprise that many of last years suicide attacks in Afghanistan were poorly carried out. On further investigation, if was found that al Qaeda had to rely on the mentally ill, or mentally deficient, "volunteers", for there were no, more capable, Afghans willing to do the suicide bit. This is one reason why Arabs are still being brought in for "martyrdom" (suicide) missions.
The Afghan al Qaeda is also having trouble building the support infrastructure for suicide attacks. A number of the suicide bombs have not gone off, because expert bomb builders are not available. In some cases, the police caught the bomber because the support team members were not experienced enough to get their man to the target, and avoid detection and capture along the way. This infrastructure is not easy to build, or sustain. In Pakistan, the suicide bomber campaign against the government resulted in a huge counterattack. This was especially the case after al Qaeda made several attempts to kill Pakistani president Musharraf. The Pakistani police killed or arrested most of the suicide bomber support crew, and that led to a sharp decline in bombings. Same thing happened in Israel, in Iraq and, soon, in Afghanistan as well.