In order to answer this we must look at how Marx envisioned the society after capitalism and what was the state of economy of the USSR at the beginning (or the state of the Russian Empire at the end of the regime).

Why economy? Because for Marx this was one of the most important aspects of a society and the economic model a givens society has, influence the rest. Marx saw capitalism as a phase in the development of society which was destined to be replaced by a socialist one. This transition, Marx argued, will happened in the most developed (industrialized) countries.

Russia was one of the least industrialized countries among the great powers and while the last decade of the Empire saw a rapid development, it was still backward. The Russian communists tried to present an explanation on why communism could be achieved in a non industrialized country but at the end communists needed to do something that Marx didn't really predicted: not only take over the factories, but also expand industry, transports, communication etc. Russian communists needed not only to lead the proletariat but create much of this class.

When we think about USSR's industrialization we think of the Stalinist years and the 5 year plans and it is partially right because during his time in power this happened, but the general idea of industrialization was nothing new. As mentioned above the czarist regime started it, Lenin alluded to it as the slogan " Communism is Soviet power plus the electrification of the whole country" shows. Stalin in a way just continued the trend with some specific characteristics.

The old regime wanted industrialization mainly because this was the way to preserve the status of a great power for Lenin and especially Stalin, industrialization meant that but also took on a ideological aspect as the workers were the party's base (in ideology and mostly in practice).

Why the Stalinist emphasis on heavy industry and not light industry? Because in a way it makes sense as heavy industry is necessary for other branches: steel production, machine production are necessary for further industrial expansion. Chemical industry is needed for a whole variety of things (like dyes, fertilizers for agriculture), you need to build huge powerplants to power all this.

Another reason why Stalin focused on heavy industry is his belief that USSR was surrounded by enemies and it will be attacked in the future. Heavy industry is necessary for building a powerful military and that he did during the 30's.

So, bottom line, focusing on industrialization and especially heavy industry was not something that PCUS came with as it had a already started (the Putilov factories had 10.000 workers before WW1) but they put a lot of effort because heavy industry is needed for further industrial and military development of a large country and also because now, unlike during the old regime, industrialization also became a ideological necessity.