Why are ionization smoke detectors bad at detecting smoldering fires?
The smoky fires send to have larger smoke particles reaching the smoke detector before smaller particle products of combustion. The larger particles seem to be more quickly detected by photoelectric sensors. However the actual detector performance is also influenced by the detector sensitivity and other design details. The performance of various detector designs is influenced by efforts to avoid nuisance alarms from cooking, water vapor from showers and other factors. Evaluations made by testing agencies seem to be based mostly on testing rather that theoretical evaluation of the designs. Since serious injury and death seem to result more often from smoke inhalation than flame, detection of large smoke particles is given priority by some experts. Perhaps there is a study somewhere that factors in the detection capability of human and animal noses.

Reference: NFPA and pages linked there.