In ESB, Obiwan says that if Luke goes to face Vader he will face him alone, “I can not help you”. But look what Yoda did in TLJ. Sure seems like Obiwan could have lent a hand now.
He says “If you chose to face Vader, you will do it alone. I cannot interfere” . This wasn’t that he couldn’t do anything, but the fact that Luke had to face this challenge on his own. He was being warned that he needed to complete his training, but he refused to listen and needed to face the consequences and not feel that Obi-Wan could just come along and save his ass.
And don’t forget Obi-Wan’s line in ANH: “If you strike me down, i shall become more powerful than you can possibly imagine.” . Up until TLJ, the most we see a force ghost being able to do is interact with their surroundings by moving some twigs and sitting in a log. A force ghost being able to use that power has been there since day one.