The reason for clubbing these two is because these two show a lot of common geography and common references.
The Kinnaras and Kimpurushas are the most left out people when it comes to references. The primary reference of them comes when the Puranas describe the geography of Jambudweep saying that there is a country called Kimpurushvarsha lying beyond the Kailasa where the Kinnaras and Kimpurushas live.
The Kimpurushas thus have a fixed territory of Kimpurushvarsha but the Kinnaras are also mentioned to be living along with Yakshas and Gandharvas in certain places like (Mahabharata 3.139).
Thus the exotic tribes were quite intermixed and lived in common areas and kingdoms. The major places of Kinnaras and Kimpurushas were Gandhamadan and Mandara mountains to the north of Kailash mountains and to the east of Meru. The Ramayana also mentions that along with the Yakshas and Rakshasas, Gandharvas, Kinnaras and Kimpurushas were also living in Lanka under Kubera.
The etymology of Kinnara and Kimpurusha is quite same owing to the fact that the Vedic people thought that they were half men and half lion. This happened because the Kinnaras and Kimpurushas were quite cut-off from the Vedic civilization in mainland India.In some places, the Kinnaras are mentioned to be one of the sub-tribe within the Gandharvas ( MBh 2.10).This shows the intermixing within them. All these tribes are clearly under the sway of Kubera since in many places in Mahabharata, it is mentioned that Kubera was their head.
The main reason why one remembers the Kinnaras is their phenomenal ability to sing beautifully. Mahabharata says that once you venture into Kinnara territory you will find beautiful Kinnara women (Kinnari) along with female Vidyadharas singing sweetly. Arjuna also had taken the form of a Kinnara to teach music to Matya king's daughter.Thus the Kinnaras are always remembered of having sweet voices.This also gives us the clue of why these people were closely related to Gandharvas and Apsaras since even the later two were involved in music and dance. Bhagvata Purana says that Ram was worshiped in the Kimpurushvarsha.