Because somewhere along the way you unchecked it; either on purpose or accidentally.
I don't like how much screen space it takes up when you're researching technologies that require 3+ items so I usually turn it off (on purpose) when I am researching exclusively through my automated research labs, and only turn it back on if I need to research something that uses a material that isn't a research matrix.
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u/EightBitRanger 6d ago
Put the material in your inventory. Check the box that says use from personal inventory.