One eighth note,
two sixteenth notes

½ a beat,
¼ of a beat,
¼ of a beat 

This figure has appeared around the Middle Ages.
It was the first possibility derived from the master figure.
This is easy to understand since the omission of the second sixteenth note, already the weakest of the group, only serves to reinforce the first.

A: Start by doing this rhythm, using the syllable "Ta".

B: Transform the second "Ta" into a "Tu".

C: Eliminate the "Tu".

One long, two short