This is a case where the universal custom is according to the Arizal.
In Halacha, the Mitzvah of Tefillin shel yad is to bind it to the bicep and then wrap around the fingers three times. The three fingers do not invalidate the binding though. Therefore, according to halacha, one would put the Yad and immediately put the Rosh and only afterwards wrap around the finger.
The Arizal on the other hand holds that the seven circuits around the arm is an intrinsic part of the Mitzvah. He holds this for Kabbalistic reasons. Therefore, according to him, you must complete the wrapping of the 7 circuits before putting on the Rosh.
The universal custom is according to the Arizal.
An interesting question is if the custom was this way before the Arizal and he only gave it a reason or if the custom developed according the the Arizal's teachings. I have never looked into that.