I was looking around on Google Earth and saw that St Petersburg is called St Petersburg, and the direct area around it is to, but the region itself surrounding it and the larger area is called Leningrad Oblast. When the city was renamed, why wasn't the oblast changed as well? Apologies if this has been answered before. I checked thoroughly, but I couldn't find it. I might just be blind though.
The name of the city was changed to Saint Petersburg in 1991 for a variety of reasons:
The city, much like Moscow itself, tends to be much more "liberal" (in a very broad sense of the term) than smaller cities or rural parts of Russia. This is as much true today as it was during the end of the Soviet era (if not earlier, but regions having distinct politics would have been anecdotal at best before Gorbachev's reforms).
Specifically with then-Leningrad, from 1990 on the city was governed by Anatoly Sobchak, who from 1991 created the post of mayor. Sobchak was a "liberal" at the time, being a political ally of Boris Yeltsin and favoring greater reforms and openness to the West that would dismantle communist institutions. I put "liberal" in quotes because these things are all relative terms - Sobchak was also mentor to Vladimir Putin and got him involved in the city administration early on. In particular, Sobchak held a nonbinding referendum in June 1991 on the status of the city's name, specifically if the city should revert to its pre-World War I name. A majority of voters approved the measure.
Apparently not up for debate were reverting the name to its World War I era name ("Petrograd"), nor adopting an idea put forward by Solzhenitsyn of coming up with an entirely new, extra Russian-sounding name: "Svyato-Petrograd". There also was a more complicated idea floated by the city council to just rename the historic downtown Sankt-Peterburg, and keep the rest of the city Leningrad. Gorbachev openly opposed the renaming, but regardless of that 55% of voters approved the change.
The name change itself didn't actually take effect until Sept. 6, and by that point the political scene had changed drastically, with the August 1991 coup attempt put down, most Soviet Republics declaring independence, and Yeltsin taking greater power and banning the Communist Party. Changing the city name at this time was an opportune move (and by the way, Russian Communist leader Zyuganov has consistently complained about the name change basically to the present day).
For what it's worth, a similar process happened at the same time in Yeltsin's hometown and Russia's fourth largest city: Sverdlovsk, now Yekaterinburg. The city council there also voted in Sept. 1991 to rename the city to its pre-revolutionary name, but the oblast (province) that Yekaterinburg is located in is still named Sverdlovsk.