Rails and Django emerged at a time when RDBMS ruled, and they were both built around an ORM. Now you have more database options that have less of an impedance mismatch, but because Rails and Django are so ORM-centric, forgoing the RDBMS in favor of another option means you now have a framework mismatch.
And Rails & Django were (at the time) also built around a specific ORM. Compare this to Catalyst, which also came out same time as Rails & Django, was built from the ground up with an agnostic model and thus could use any ORM available or easily adapt to different data models.
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Rails and Django emerged at a time when RDBMS ruled, and they were both built around an ORM. Now you have more database options that have less of an impedance mismatch, but because Rails and Django are so ORM-centric, forgoing the RDBMS in favor of another option means you now have a framework mismatch.
And Rails & Django were (at the time) also built around a specific ORM. Compare this to Catalyst, which also came out same time as Rails & Django, was built from the ground up with an agnostic model and thus could use any ORM available or easily adapt to different data models.