Preserving user privacy is paramount when it comes to publicly disclosed datasets that contain fine-grained data about large populations. The problem is especially critical in the case of mobile traffic datasets collected by cellular operators, as they feature elevate subscriber trajectory uniqueness and they are resistant to anonymization through spatiotemporal generalization. In this work, we investigate the $k$-anonymizability of trajectories in two large-scale mobile traffic datasets, by means of a novel dedicated measure. Our results are in agreement with those of previous analyses, however they also provide additional insights on the reasons behind the poor anonimizability of mobile traffic datasets. As such, our study is a step forward in the direction of a more robust dataset anonymization.