وَبِـ«أَيَّانَ» لِزَمَانِ الْمُسْتَقْبَلِ
Ayyān asks for future time.
Matā can refer to past or future, whereas the source describes ayyān as relating specifically to the future. Consequently they are not interchangeable in every setting.
Ayyān is also especially associated with events of magnitude or gravity. That stylistic association gives the question a distinctive rhetorical tone which is lost if both ayyān and matā are translated simply as 'when.'