Just curious why the vast majority of Rimfire rifle owners use scope rings instead of one-piece scope mounts on picatinny rails?

Price? Size of scope?

you can usually get rings lower and they're lighter.

IMO, mounts only make sense if you want to be taking mounts on/off all the time, want more height, and/or want some elevation offset.

Mauser-style turn bolt rifle actions dominated everything for over 100 years.  Most turn bolt rimfires fit that pattern.  

The AR-15 evolved and began to gain wide acceptance as a general purpose sporting rifle only in the 1980's and 1990's.  

Mounts evolved , in large part, because of inadequate room on top of the AR-15.  Given the long eye relief and length of conventional scopes, the rings did not allow placing the scope far enough forward.  The straight stock required extra height above the bore (note differences in iron sight heights, too)..

To help with this, Leupold came out with a short eye relief scope meant just for the AR but most did not want to buy a new scope.

The one piece mount provided the vertical height needed and moved the scope forward.

Turn bolts and the AR had different evolutionary paths.  Will they merge over time?

For those still curious, its considered best practice to mount the scope as low as possible on bolt action rifles, including rimfire.  This reduced lateral POI error due to canting the rifle, and simplifies holdover calculation.  It also places the eyepiece of tbe scope in line with the typical buttstock comb on most bolt guns intended to be used with scopes.

One piece Picatinny rails or other one piece mounts can also block the ejection of the spent case, as would have been a problem on this Remington 541-S bolt .22LR.  This a two piece separate front and rear bases and rings made by Leupold.  Plenty of room for ejection.