Russia is big. They cannot cover the whole country so they prioritize what they think are high value/risk targets by moving protection to those places leaving less risk places undefended. In turn Ukraine is looking for places not protected.

Note that this factory was thought to be beyond Ukraine's reach. So considering it very low risk and thus not worth protected seemed like the right choice. Only in hindsight do we know it is in reach and thus not very low risk. Even now it is hard to know how Russia should assign risk.