This can be easily answered by using only the Durga Sapta Sloki as the reference,which is a part of the famous Devi Mahatyam found in the Markandeya Purana.
The Durga Sapta Sloki (Seven verses to Durga) was taught by the
Goddess herself to Lord Shiva, to realize our objectives in the Kali
age. This is also called Amba Sthuthi. These seven verses are a
selection from the Devi Mahatmya. It is believed that reading these
verses is equivalent to read the full text of Devi Mahatmya.
śiva uvāca |
devī tvaṃ bhaktasulabhe sarvakāryavidhāyinī |
kalau hi kāryasiddhayarthamupāyaṃ brūhi yatrataḥ ||
Oh Goddess who can be easily approached by devotion, who helps us to
get all actions done. Please tell me the trick of achieving result for
these actions in Kali age.
śṛṇu deva pravakṣyami kalau sarvaiṣṭasādhanam
mayā tavaiva snehenāpyambāstutiḥ prakāśyate ||
Oh god, please hear what I say, which is the one fulfilling all
desires in Kali age. Due to my love for you, I am telling you this
prayer to Mother Goddess.
om asya śrīdurgāsaptaślokīstotramahāmantrasya
nārāyaṇa ṛṣiḥ । anuṣṭupādīni chandāṃsi ।
śrīmahākālīmahālakṣmīmahāsarasvatyo devatāḥ ।
śrī jagadambāprītyartha pāṭhe viniyogaḥ ॥
Of this Durga Sapta Sloki, Narayana is the Rishi, Anushtup (eight
syllables in a pada) and others is the meter, Goddess Mahakali,
Goddess Mahalakshmi and Goddess Mahasarasvati are the deities and it
is recited for the love of Shri Jagadamba (Mother of the Universe).
So,when you see that the Devata of this Durga mantra are three Goddesses Mahakali,Mahalakshmi and Mahasraswati, it easy to conclude that Jagadamba Durga(to please whom this mantra is chanted) is a combined form of all the three aforesaid Goddesses and thus is superior to each of them considered individually.
So,that's basically the reason why Durga Devi is given more importance than other forms of Devis or in your words "only associated with Devi."
And,btw Devi Durga is not only a combined from of various Devis but she is a combined form of various Devas also.