The point was that Trump has a very short attention span and the regular channels weren't working to capture his attention, so the bad actors involved needed to make Trump mad in order to get him to act.
It all comes back to loyalty and a Fraud Guarantee.
Igor Fruman and Lev Parnas are two Soviet-born real estate businessmen who were hired (via their company, Fraud Guarantee, Inc) by Rudy Giuliani to find some piece of information that could counter the Mueller investigation's damage to Trump's brand.
While they were digging, they hit an obstacle in the form of Marie Yovanovitch. Not that she specifically knew anything or was specifically causing them any issues, but she was the only career diplomat (ie, not picked specifically by Trump and thus of questionable loyalty to Trump, rather than the United States) in Ukraine at the time, and thus would have served as a challenge to their ability to quickly use US government channels in their search for political capital for Trump to use. So they decided to remove her and find someone who would aid their search rather than hamper it.
In April of 2018, they asked Trump directly to fire Yovanovitch. Trump mentioned it once to Secretary of State Mike Pompeo, then forgot.
On May 8th 2018, they met with then-Representative Pete Sessions and donated $325,000 to the America First PAC earmarked for his campaign, and promised to raise $20,000 for his campaign directly. On May 9th, Sessions and the two gentlemen appeared in photos together at the Capitol. At some point before the end of June 2018, Pete Sessions wrote a letter to Pompeo telling him that Sessions had heard from Kyiv associates that Yovanovitch was badmouthing Trump in private, and should be removed. It appears that Pompeo didn't pass the message on to Trump for whatever reason.
Pete Sessions loses his reelection campaign despite the contributions in November 2018, but Trump has not yet removed Yovanovitch. Fruman and Parnas then offer to pull strings to get Sessions the Ambassadorship if he can further push the issue of ousting Yovanovitch, as working around her is indeed proving to be a major delay in their plans. Also around this time, Ukrainian officials tell Yovanovitch that the former Ukrainian Prosecutor General (and guy Biden bragged about firing for being corrupt) Lutsenko had been in contact with Rudy Giuliani and that one of their goals was to remove Yovanovitch from her post.
In February 2019, Ukrainian officials become aware of Fruman and Parnas' activities and alert Yovanovitch of this, as well as their connection to trying to get Yovanovitch fired.
In March, Conservative columnist John Solomon posted an interview with the corrupt former prosecutor Lutsenko that claimed Yovanovitch was actively blocking investigations into corruption, and gave a fraudulent list that he alleged she gave him telling him not to investigate Burisma, the company Hunter Biden worked for (long after the alleged fraud that they wanted investigated). After that interview was published, everything blew up. Sessions, Giuliani, Hannity and others all took to Fox News to decry Yovanovitch's alleged unpatriotic behavior. Donald Trump Jr. retweets one of their comments, and it's apparently from this retweet that Trump finally takes it to heart. Giuliani gives Trump a nine page document of all the alleged crimes Yovanovitch was up to, labeled her a 'Never-Trumper' and tied her to Joe Biden.
By the end of April, Yovanovitch was flying back to DC.