Plant families like Convolvulaceae and Solanaceae are included in the order Polymoniales mainly on the basis of:
The plant order Polymoniales is a higher taxonomic category that aggregates families exhibiting specific shared traits. According to NCERT, families such as Convolvulaceae and Solanaceae are grouped into the order Polymoniales predominantly based on their floral characters, which serve as a stable aggregate of features for higher-level plant classification.
Ref: NCERT Class 11 Biology