The answer to the question in the title of this article is that Russiagate was created by CIA director John Brennan. The CIA started what is called Russiagate in order to prevent Trump from being able to normalize relations with Russia. The CIA and the military/security complex need an enemy in order to justify their huge budgets and unaccountable power. Russia has been assigned that role. The Democrats joined in as a way of attacking Trump. They hoped to have him tarnished as cooperating with Russia to steal the presidential election from Hillary and to have him impeached. I don't think the Democrats have considered the consequence of further worsening the relations between the US and Russia.
Public Russia bashing pre-dates Trump. It has been going on privately in neoconservative circles for years, but appeared publicly during the Obama regime when Russia blocked Washington's plans to invade Syria and to bomb Iran.
Russia bashing became more intense when Washington's coup in Ukraine failed to deliver Crimea. Washington had intended for the new Ukrainian regime to evict the Russians from their naval base on the Black Sea. This goal was frustrated when Crimea voted to rejoin Russia.
The neoconservative ideology of US world hegemony requires the principal goal of US foreign policy to be to prevent the rise of other countries that can serve as a restraint on US unilateralism. This is the main basis for the hostility of US foreign policy toward Russia, and of course there also is the material interests of the military/security complex.
Russia bashing is much larger than merely Russiagate. The danger lies in Washington convincing Russia that Washington is planning a surprise attack on Russia. With US and NATO bases on Russia's borders, efforts to arm Ukraine and to include Ukraine and Georgia in NATO provide more evidence that Washington is surrounding Russia for attack. There is nothing more reckless and irresponsible than convincing a nuclear power that you are going to attack.
Washington is fully aware that there was no Russian interference in the presidential election or in the state elections. The military/security complex, the neoconservatives, and the Democratic Party are merely using the accusations to serve their own agendas.
These selfish agendas are a dire threat to life on earth.