USSR spent a lot of resources to prevent wood fires, plenty of people worked on that. Russia significantly cut all investitions to that area, infrastructure is aging, people are untrained, corrupted and there are few of them. It's not a surprise that those fires every year are getting bigger. It'll get worse with time.

Another contributing factor is wood stealing. High-ranked officials are selling wood illegally, mostly to China and then covering missing wood with fires.

Also some laws are outright stupid. In the past local people were gathering fallen trees. Recently there were some laws which made this practice much harder, some people were even convicted for that. So people stopped to gather fallen trees. And fallen trees is another contributing factor which increases chance of putting forest on fire.