Faster assays make good financial sense. Screening materials are a significant fraction of the total screening costs. In many cases, the costs of the chemical and biological reagents can add up to three-quarters of the total screening costs. Miniaturization is an attractive way to hold down material costs. Also, miniaturized tests can help reduce the need for laboratory space.
Automation can help with speed and reproducibility of results.
Now that companies are using microplates with 384 or 1536 wells, they've passed the physical limit at which humans can prepare the plates. If an assay can't be automated, then it won't be feasible in the HTS environment. The sensitivity of some detection methods is directly related to the volume of the sample, so smaller samples lead to diminished sensitivity. If scientists must increase the detection time to compensate for smaller sample volumes, then the method is not a good candidate for use in HTS systems. In addition, some of these radiometric tests require long detection times or large sample volumes.