Why does Śrī Raghunātha, being a Yugala-upāsaka (worshiper of Rādhā-Kṛṣṇa), worship only Śrī Rādhā?

Śrīmat Sanātana Gosvāmī provides the answer in his Bṛhad Bhāgavatāmṛta (2.7.11):

śrī rādhājñā pratipālanenaiva śrī kṛṣṇasya vaśīkaraṇāt svayam evādhikādhika tat saṅga sukha saṁsiddher iti dik:

“By serving Rādhā or following Her order, Śrī Kṛṣṇa is more easily captured than when one serves Him directly, and the bliss of His company automatically becomes more and more available.”

In Viśākhānandada Stotram (131) Śrī Raghunātha dāsa writes:

bhajāmi rādhām aravinda-netraṁ
smarāmi rādhāṁ madhura smitāsyām
vadāmi rādhāṁ karuṇā bharārdraṁ
tato mamānyāsti gatir na kāpi

“I worship Rādhā, Who has lotus like eyes, I remember Rādhā, who has a sweetly smiling face, and I speak of Rādhā, Who is melting of compassion. In this way there is no other goal for me”.