I found an interesting post on the Truth in Religion & Politics blog that asks and answers the question.
What is so unique about the earliest disciples of Jesus being martyred for their claim Jesus was raised from the dead? Many believers of various religious systems–Muslims for example–die and commit suicide regularly for what they believe to be true. Christian apologists arguing for the historicity of the Resurrection use the fact that Jesus’ disciples and subsequent followers allowed themselves to be killed, without recanting their conviction that Jesus was raised from the dead. Is this line of reasoning valid? Does the fact that others die willingly for their religious faith undercut the veracity of the argument for the Resurrection?
The most important aspect of this detail is the historical proximity of the disciples to the event. The disciples were contemporaries of Jesus and the Resurrection event. They were witnesses to Jesus’ life; witnesses to His death; and claimed to be witnesses of His being alive after having been buried.
If we claim the Resurrection was a story invented by the disciples, we have to also have to claim they died for an event they knew they invented themselves.
[…]Keep in mind I am not arguing for modern or even 2nd century Christian martyrs as evidence, but rather the first disciples who claimed to be actual witnesses to the events themselves. Muslims who die in suicide attacks are not first hand witnesses to Allah, or miracles of Allah. Mohammad did not perform miracles, he claimed only to be a prophet. Given this aspect of Islam, Mohammad’s cohorts were getting their theological insight second-hand from someone who claimed to speak for God. They are not in parallel circumstances as the first martyred disciples who claimed to see with their own eyes the events for which they were killed. Muslims willingly die for what someone told them was true, and in fact they do believe the message of Mohammad is true, but they lack first hand experience of his claims; they could not necessarily have known his claims were false. Jesus’ disciples claimed to be eye witnesses to the Resurrection, they would be in the position to know their own story was false.
Lots of people die for their beliefs, but only the first century Christian martyrs were in a position to know whether they saw Jesus after his death or not.