Why Asr?
The prayer singled out for special care as the day starts to slip away.
Asr falls in the late afternoon, when fatigue and distraction are highest. The Qur'an gives it particular emphasis, and the narrations warn strongly against neglecting it.
Qur'an
"Maintain with care the prayers and [in particular] the middle prayer, and stand before Allah devoutly obedient."
Qur'an 2:238 — the majority of scholars identify the 'middle prayer' as Asr
Authentic (sahih) hadith
"Whoever misses the Asr prayer, it is as if he lost his family and property."
Sahih al-Bukhari 552