first was episode 4.if you remember, in episode 5: "go to the dagobah sistem and find master yoda, the jedi master who instructed me"
GL's first ideea was that yoda was obi'wans master
after epsiode 6, GL wanted to show how good was obi-wan, since in the first trilogy...he wasn't sooo impresionating.so that's wy qui-gon died.but obi-wan did kill maul
if yoda would be his master, it would be no really big adventure for, obi-wan....yoda was a milion times better than maul, he would kill him in two minutes or less
and maul couldn't kill yoda because yoda died in episode 6.

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"Obi-Wan is a great mentor, as wise as Master Yoda, and as powerful as Master Windu."
―Anakin Skywalker