Quoting "same spot"... short answer... don't know.

I guess I meant same latitude, as same spot would imply something slightly different. Part of the reason one sees them more commonly at the edges of the disk is purely visual... due to the contrast of having "space" as the backdrop vs. having the Sun's disk as a backdrop. I guess I'm implying that coronal cavities form more often than what we easily see, which is obviously true since coronal cavities are part of the whole package which comprise a prominence, the coronal cavity just being different detail exposed due to the different wavelengths/temperatures of each of the SDO sensor bands.

As to the physics as to why they seem to form more often at this latitude range (if indeed they DO form here more often (statistical numbers for that?))... good question.

I would guess (GUESS!) that all prominences are not created equal. That some form with well defined structure in the temperature ranges that show these cavities well. If there is statistical truth to cavities forming more often here, at this latitude, it could possibly be used to reveal some info regarding plasma flow patterns, temperature distribution, etc. of the sun and/or prominence formation.

Confused by this tripe? See line 1 above.

--