Because Russia is a 3rd rate power with an economy to match, China can pull them into thier sphere of influence as a junior partner for next to nothing, while backing the west gets nothing. The US and it's allies are already gearing up for a confrontation with China, China propping up Russia so it can be a distraction in Europe is a reasonable move to try and divert western forces.
PPP adjusted (which is the more accurate measure) they are a >$4 trillion dollar economy, which is barely behind Germany:
en.wikipedia.org
Some rankings of international militaries still have Russia ahead of China and a close second to the US:
Ranking the nations of the world based on current available firepower.
www.globalfirepower.com
They have just focused their military on land to control their borders and deal with nearby threats in Europe. The US is more focused on the navy and air force than their ground forces, while Russia is more focused on their army and supporting air force:
Gresham’s Law states that bad money drives out good money, but anyone who has spent time around Washington, D.C., knows that this law can safely be
warontherocks.com
People don't think about PPP adjusted budgets as the nominal spending in dollars doesn't really tell you what can be actually be bought for that budget. So fighting Russia where they would have to be fought, their backyard, actually leaves the US military at a severe disadvantage. Not only that, but Euro NATO has demonstrated they are effectively useless in such a war as would be required to beat Russia, as the core of it, Germany, is completely impotent, while France and Britain are only set up to fight insurgents abroad, so their spending is on things useless to fighting the Russians.
Far from being a junior partner Russia is quite formidable an ally on its own especially because if push came to shove China couldn't function without Russian resources in the event of war with the US.