Because cylinder head studs offer more clamping pressure than bolts offer. If you have a high compression engine and the block is only 4 bolts per cylinder then you might have a problem with head gaskets. That problem could be fixed if allowed head studs. Under the old regime at NHRA, that's what they would tell you. Under the technically challenged new NHRA, they may allow it, not fully understanding the ramifications to fellow competitors.
Why do you want head studs if they offer no advantages?
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