According to ????? ?????? the reason is as follows: There is a segula to look for a letter from your name in the part of the sefer Torah being displayed at hagbah, so people skim the text with their finger, but it’s rude to point at something as holy as sefer Torah with your index finger so you use your pinky which is apparently more aidel.
I only know this because I was flipping through the sefer in my local shtibel a while ago. I didn’t know there were halachos about which finger to point with outside the beis hamikdash. When I lein on weekdays (including this morning) I usually use my middle finger since it’s hard to point with any other one with my tefillin on.