57% of ethnic Russians are Latvian citizens, but to become a citizen you have to go through a naturalization process which involves testing on knowledge of the Latvian language, as well as Latvian history, constitution and other things. It sounds strange, but not many Russians lived in Latvia prior to the 20th century (with the exception of the "Old Believer" type of Russians who thoroughly intermarried with Baltic ethnic groups by then) and especially after the invasion of 1940 where Latvia was transformed into a SSR and tens of thousands of Russians immigrated to Latvia and were heavily subsidized in various manners, including the Soviet Union building them their own towns. Also during and following World War Two, despite having a population of less then two million people (including Old Believer Russians and Germans for example) almost 300,000 Latvians were deported or expelled from the country. And until 1991, the Latvian culture, most specifically its language, was suppressed by the Soviet authorities as every child was intended to be taught Russian first, and all official paperwork, signage and news in the Latvian SSR was to be Russian.
One of my Estonian friends used to tell me that whenever they spoke Estonian in public, Russians were tell them to stop speaking a subhuman language or to speak a human language. So ultimately, when 1991 came around, you have significant minorities of Russians who refused to speak Latvian or Estonian or what have you mostly out of spite, and to this day, a fair number still do. However it's also important to note many Russian-Latvians also supported Latvian independence as well. Another factor today is that the Russian Federation continues to make a point of stating that they feel the 1940 Invasion and subsequent occupation of Latvia was wholly legitimate, accused the Baltic governments and people of being in collusion with the Nazis (despite the complete lack of resistance to the 1940 Russian invasion or the fact that Soviets had basing rights in the Baltic Republics.... sound familiar? ) and refuse to still call the period between 1940-91 an occupation.