Okay so *je ne suis pas sûr pourquoi...** isn't grammatically correct, but I'm going to go on a limb here and assume your intention was to translate the English I'm not sure why...
Expressions that would translate this in French would be,
Je ne suis pas sûr de comprendre pourquoi / je ne comprends pas pourquoi (when expressing confusion, as in this case)
Je ne suis pas sûr de savoir pourquoi / je ne sais pas pourquoi (when expressing simple lack of knowledge without connoting confusion)
Now for the real question : subjunctive or indicative? In this case, the subjunctive doesn't work, because you're asserting that people are going to the concert, even though you don't know why they're going. The fact that the large number of people attending is a reality requires indicative.
You can also use the conditional to express that the situation is a hypothetical that is being doubted, eg:
Je ne suis pas sûr de savoir pourquoi il y aurait beaucoup de monde, vu comme il chante (I don't see why there would be many people there, given how he sings)