I'm guessing the bottleneck lies in surface area to effectively collect samples rather than the effort to inspect samples and processing them to determine origin. The roof is going to have contaminants, sure, but so is any other collection area. Using a magnet to get iron based particulate then picking out rocks with unique elements is likely a lot cheaper than a purpose built facility to collect micrometeoroid samples with reduced contamination
Yeah it was the fact that they could power wash the roof clean and then use it as a collection mechanism, and it was high enough to be isolated from other sources of dust.
