I didn't, I signed up to an account and stopped at this point because unlike a financial institution they are not governed by laws that require this or would ensure the security of such data.

But I totally understand why people did supply it.

They supplied it because:

1) They've been trained to

2) For a while MtGox was actually the most well-known (and therefore trusted) of the exchanges

On #1, every financial institution in the world goes through some form of KYC (Know Your Customer) anti-laundering process that involves identity verification. And people are so used to doing this when signing up to financial accounts that they failed to comprehend that MtGox (and other exchanges) is not a financial institution being regulated by the same safeguards for their data.

On #2, even if you accepted that they were not a financial institution almost all of the bitcoin exchanges are asking for verification. And the larger more well-known exchanges are viewed as "least risky". It's a herd-like mentality, if these thousands of others did it, it must be safe.

I never supplied the info, to this or other exchanges, but I totally see why excited people who feared missing out as the price rose were willing to do so.