The nuclides of interest are either noble gases which due to their inert behavior are inherently not trapped in a crystallized mineral or has a short enough half-life such that it has decayed since nucleosynthesis, but a long enough half-life such that it has built up measurable concentrations.
Nuclides of interest are either noble gases, which are inherently unreactive and therefore not trapped in minerals, or those with a short half-life that has decayed since the formation of the universe, but a long enough half-life to have built up detectable amounts.