Heh.
To be fair, you can use parrafin to make some compunds that might be able to act as detonators, but it's iffy. It's advocated as an improvised measure, but less reliable than actual detonators using primary explosives.
Of course, drilling all those holes would have been rather obvious. Becuase if, especially as Christophera has claimed, the explosion was distributed, you'd have to drill a lot of holes. If you have C-4 in a solid column ground to top, and you only put a detonator in the bottom, it isn't all going to go off at once and I'd suspect much of it wouldn't detonate at all. You'd have to place detonators at various points all along the length, at least one per floor, for that single c4 column.
Not to mention the difficulty in knowing where to drill, as well as making sure you don't drill into your explosive. C4 is pretty stable, but becomes less so as it ages, and while unlikely, you can't discount the probability fo accidentally setting it off from a hot drill bit or (if you accidentally drilled into the rebar or the steel column) sparks (combined with the pressure produced by the drilling).
