Simple question with many answers. It really comes down to who thought what was valuable. Many national bodies microfilmed drawings because microfilming is (was?) a standard archival process. A single roll of 35mm film will hold about 1,000 drawings, not much different than a CD-ROM with scans.
Mosts original drawings where created as reproducibles so that copying processes such as blueprinting, ozalid etc could be used to generate exact copies for distribution. Many, many originals have ended up in the trash. Too much room to store, poor condition, not important anymore etc. etc. This, of course doesn't account for the drawings destroyed during war, fire and flood.
Truely original drawings are very rare, usually you get a many generation old copy.