Mueller understood his mandate was to investigate before the fact conspiracy between Russia and the Trump campaign. When people allege "collusion" between Trump and Russia, they are not talking about a before the fact conspiracy, but rather tacit and sometimes explicit coordination between the two groups to benefit one another.
Mueller kept his investigation very narrow in that sense, and failed to pursue various avenues of investigation, including not even looking at any financial ties between Trump and Russia (of which there are many, as confessed by Eric Trump). In fact the Mueller Report notes that the Trump, his campaign, and his associates obstructed the investigation by lying and destroying evidence. So a finding of insufficient evidence doesn't really put the issue to bed. Mueller didn't even interview Trump in person, and Trump lied to Mueller in his written responses to questions.
In fact, the Republican-chaired Senate Select Committee on Intelligence released a later report which expands on the Mueller probe, and it more or less confirms what those who alleged "collusion" were saying all along. Remember, the original position Trump took was that there were zero contacts between his campaign and Russia. No contacts, no deals.
The Mueller Report and later the SSCI report lay out over 100 contacts between Trump and Russian intelligence and government officials. In particular:
- Trump's son, son in law, and campaign manager met with a convicted Russian Spy in Trump's house on behalf of the Russian government. The Trump Campaign was very eager to meet with her. The spy laid out specific terms: in exchange for dirt on Hillary Clinton, they expected the relaxation of sanctions levied by the Magnitsky act.
- Paul Manafort in fact turned over internal campaign data to a Russian intelligence officer. From the Senate Committee: "The Committee found that Manafort's presence on the Campaign and proximity to Trump created opportunities for Russian intelligence services to exert influence over, and acquire confidential information on, the Trump campaign," ... "The Committee assesses that Kilimnik likely served as a channel to Manafort for Russian intelligence services, and that those services likely sought to exploit Manafort's access to gain insight in to the Campaign".
- In the trial of Roger Stone, we learned that Stone actually gave Trump advance notice to Trump of hacked DNC e-mails, as Stone was in direct communication with Julian Assange. Trump had previously been briefed by the FBI about foreign interference in the campaign, and he failed to alert the FBI of the incoming hacked information. In fact he kept quiet about this, and instead not only did he trumpet the hacked e-mails at every opportunity, he actively encouraged the hackers to try to obtain more information.
- From Michael Cohen we learned that Trump's claims of having no active deals in Russia was a lie, when in fact Trump was attempting to get a tower built in Moscow, complete with a penthouse gift for Vladimir Putin. It was reported by Buzzfeed that Trump instructed Cohen to lie about this, and while Mueller disputed this, we later learned that in fact when Cohen lied to Congress about the existence of this deal, he was doing so with the understanding that it was Trump's wish for him to do so. Whatever Cohen's motivation though, we do know Trump wanted to keep it a secret from the American people and he himself lied to all of us about the existence of a deal.
- Fast forward to Trump's actual presidency, and he has done everything he can to show deference to Russia, from pushing for Russia to rejoin the G8 (from which they were expelled for the invasion of Crimea), to making blundering strategic decisions in Turkey to Russia's benefit, to siding with Putin over the assessment that Russia was not responsible for hacking the DNC, to pushing the idea that in fact it was Ukraine that was responsible for hacking the DNC.
And after all of this, there still has not been a rigorous accounting of Trump's financial ties to Russia, of which there are many, and there has not been a counterintelligence investigation into the relationship. Not by Mueller, not by the House, the Senate, the FBI, or any other body. Given all of the above and everything we know about how Trump operates.
So was there a conspiracy between the Trump campaign and Russia? No, probably not, and this was not alleged by most of us who had an issue with the relationship between Trump and Russia. Was there collusion? I think the Manafort incident alone proves yes. Trump's campaign manager hands over internal campaign data to what the Senate Select Committee on Intelligence characterizes as a "Russian Intelligence Officer", while the GRU is hacking the DNC and targeting voters with a psyops campaign. Yeah, that's collusion.