Lithuania kept immigration low by economic development policies. Not only Lithuania has not one but three big cities (Vilnius, Kaunas, and the port Klaipėda). Other big towns were distributed rather uniformly as well (since the times of Marienburg rights), with Šiauliai, Panevėžys in the North, and Alytus, Kapsukas (now Marijampolė again) in the South, etc. New factories were distributed across the country just as uniformly. In particular, the Sniečkus government built agro-chemical plants in Jonava, Kėdainiai (rather central); a petro-chemical plant in Mažeikiai; a big power plant in Elektrėnai, etc. Workforce for those factories could gathered from neighbouring villages; there was no need for "friendly" immigration. Only construction of the Ignalina nuclear plant required a significant influx of Russian-speaking specialists.  

The policies meant that overall industry grew rather slow in Lithuania; there were few industrial wonders to brag about. At the breakup in 1990, Lithuania was still a very agricultural country, with over 30% of population still rural. But the upside was, of course, minimal minority worries.