It "was" nice of you to ask me to make a speech

Snoopy's received a invitation letter from the Daisy Hill Puppy Farm to ask him to make a speach. Snoopy replies, "It was nice of you to ask me..."

I wonder why he says "It was nice...", but not "It is nice..." What's the difference between "was" and "is" ? I know the invitation letter was sent to Snoopy a few days ago. At that time (a few days ago) the Puppy Farm asked him to make a speech. However, I think the action of asking is still going on (in my grammar of Chinese language), and it's hard for me to understand the difference.
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My English is poor. If there is any grammar errors above, please let me know. I appreciate your help.