Complex sentence: Large collections of samples representing tens or hundreds of thousands of individuals are necessary to conduct these kinds of studies, so researchers may perform such studies only with large numbers of samples, so many researchers struggled to acquire sufficient samples prior to the advent of biobanks.
Split into simpler sentences: Large collections of samples representing tens or hundreds of thousands of individuals are necessary to conduct these kinds of studies, so researchers may perform such studies only with large numbers of samples. Many researchers struggled to acquire sufficient samples prior to the advent of biobanks.