They did not really sell like crazy.
They weren't even available in most parts of the world and with most carriers.
Apple just received considerable media hype.
The 1st generation iPhone was in fact the only Apple product I can remember that Apple (had to)
adjust pricing down within weeks after release, with Jobs
publicly "apologising":
"I have received hundreds of emails from iPhone customers who are upset about Apple dropping the price of iPhone by $200 two months after it went on sale."
👉 When has Apple ever dropped a product's price two months after going on sale in the U.S.?
And why? Crazy sales?
Overseas sales were described as "sluggish" (see sources Wikipedia)
The 1st gen iPhone certainly was a moderate success (following an intense early adopter hype that rather soon waned off). But the iPhone's success story only really took off with the advent of the App Store.