We don't know for sure, just as we don't know for sure why he officially renamed the Jurchens to the Manchus. He never really issued a statement explaining his motivations, just made the proclamations and was done with it. There have been two competing positions on the issue, albeit more focussed on the Jurchen -> Manchu change rather than the Jin -> Qing change, centred around two historians' approaches to the issue: Pamela Crossley in A Translucent Mirror takes the view of the imperial centre and sees the renaming of the Jurchen people and Jin state as a case of political pragmatism, establishing a new and distinct political identity that could be used to encompass several non-Jurchen-speaking peoples; whereas Mark C. Elliott in The Manchu Way, taking a more bottom-up approach, argues that these renamings were part of a push to consolidate existing identities by on the one hand giving this identity a new and more recognisable shape, while also insisting on continuity from the past, notably by retroactively referring to the Jurchens of the 11th-16th centuries as Manchus. In short, the decision to rename the Jin state can be read as an attempt to distance the new state from its medieval predecessor in order to assert a new forward direction – the point of contention being whether this was to be more inclusive of non-Manchus or to more strongly emphasise the new Manchu resurgence.

Why 'Qing', though? I've encountered one or two people asserting that the state's new name of daicing would constitute an extended pun on the Manchu or Mongolian word for 'warrior'. I'm sceptical of this as the Manchu for 'soldier' is dain i haha, but Mongolian for 'warrior' is indeed daichin, so the possibility cannot be discounted. Alternatively, it's been suggested that 清 Qing, which carries the three-dotted radical for water, would confer cosmological benefits when fighting the 明 Ming, whose character contains the sun and thus is associated with fire, which in the wuxing elemental scheme is of course extinguished by water. As 金 Jin is the character for metal, which in the wuxing scheme is melted by fire, going to war under that name would be decidedly inauspicious. Given that Qing armies quite frequently made decisions around magic or cosmological signs well into the 19th century, this is not implausible. Still, on this count we can only speculate.