I'm not an expert, but as far as I know, there are several aspects to that kind of mispronounciation of "R":

1. Rarely, some people have a minor anatomical variance that does not allow them to easily roll strong "R".

2. Most often, as kids start speaking, they mispronounce all kinds of sounds. So, if the parents do not bother too much to correct (or mispronounce the sounds themselves), the kids grow up that way and when they realize they have a speech defect, it's most likely too late to be corrected. (That is most likely a case with ST.)

3. At the 19th - early 20th century, Russian culture was highly influenced by French. So much so that aristocrats were native French speakers and French was the primary language spoken at home and in the gatherings. Russian language was secondary and less of importance. As a result, Russian aristocrats used to pronounce "R" in a French-like manner. That was a fashion of the day and the chic, but nowdays it is considered a speech defect and as such is treated by speech therapists.