That's not the most objective decision in the world. If we're describing "conurbation" in specific, why not call -urb- English too, taken from the common English word urban? Urban ultimately draws from Latin urbs, but so does con- draw from com- and -ation draw from -io(n).

(In Latin, there are plenty of words ending in -atio(n); however, within the language this is not a single unit, it's a sequence of part of the verb stem plus two separate morphemes -a-t-io(n). The -at- marks the passive participial form of an a-stem verb; compare faction (zero-stem), inhibition (e-stem).)