MTHaslett said:

B) it eliminates the most painful OT moment for me when we had to watch the once noble and honorable Obi Wan use lawyer speak to excuse his manipulative lies. He didn't tell Luke the truth because he thought he could use Luke against Vader and that lying to Luke would make that easier. Watching his self-serving explanation only debases Obi Wan further.

I always saw it that Obi didn't tell Luke because he was both ashamed of his own failure, and wanted to spare the boy's feelings. Perhaps if he hadn't been killed very soon after, he would have told the truth when Luke was more ready and they had more time.