What is the reason that Obi-Wan lied to Luke by telling him that Darth Vader killed his father?

Out of universe, it's simply that Lucas hadn't written (and most likely hadn't even thought up) that particular part of the story at the time.

In universe, Kenobi was driven to obfuscate by two things:

1. he regretted his part in Anakin's downfall and couldn't own up to it to Luke

2. he didn't want Luke to follow his father

because in a certain why he wasn't lying as indicated in Return of the Jedi

Luke: Obi-Wan! Why didn't you tell me?

The ghost of Ben Kenobi approaches him through the swamp.

Luke: You told me Vader betrayed and murdered my father.
Ben: You father was seduced by the dark side of the Force. He ceased to be Anakin Skywalker and became Darth Vader. When that happened, the good man who was your father was destroyed. So what I have told you was true... from a certain point of view.

Source: IMSDB > Star Wars: Return of the Jedi

in Obi-wan's view Anakin and Vader aren't the same person and Vader destroyed Anakin. this is also in a way expressed by Padme in Revenge of the Sith when she confronts him on Mustafar

Anakin: Don't you see, we don't have to run away anymore. I have brought peace to the Republic. I am more powerful than the Chancellor. I can overthrow him, and together you and I can rule the galaxy. Make things the way we want them to be.
Padme: I don't believe what I'm hearing . . . Obi-Wan was right. You've changed.
Anakin: I don't want to hear any more about Obi-Wan. The Jedi turned against me. Don't you turn against me.
Padme: I don't know you anymore. Anakin, you're breaking my heart. I'll never stop loving you, but you are going down a path I can't follow.

Source: IMSDB > Star Wars: Revenge of the Sith

to Padme the person she was talking to wasn't the Anakin she knew

Revenge of the Sith:

Obi-Wan: And what of the boy?
Yoda: To Tatooine. To his family, send him.
Obi-Wan: I will take the child and watch over him.

A New Hope:

Luke: And if these new droids do work out, I want to transmit my application to the Academy this year.

Kenobi has been watching Luke all the boy's life, he knows Luke is intending to join the Empire to escape life on Tatooine. This will bring him into Vader's orbit, the last thing Kenobi wants, so he lies.

This is just my take on the Original Trilogy, but I think the reason why Obi-Wan lied to Luke in A New Hope, is because he didn't want Luke to go down the same path that Anakin went on; to avoid becoming a Sith. Thing is, rather than feed Luke bits and pieces about Anakin, he could've just come clean and told Luke what really happened and how he failed him as a teacher and that he was mostly responsible for Anakin's fall to the Dark Side. Not wait until Return of The Jedi, after Yoda dies of old age and THEN decides to spill the beans.

Even ignoring the fact that it wasn't written yet, Obi-Wan still has a good reason.

Obi-wan wanted Luke to fight against the Empire and The Dark Side. To this end, he might have considered it better to "Bend" the truth a little and tell Luke that Darth Vader killed his father rather than that Darth Vader was his father.

If Luke knew that Darth Vader was his father, Obi-Wan might have thought there would be a risk that Luke might (even over time) begin to feel a bit of family loyalty, and begin to sympathise with Vader a little. By telling him that he killed his father, there is no chance of this - quite the opposite.

Obi-Wan simply justified this dishonesty to himself, and later to Luke with the "...from a certain point of view" speech.