Comment by lxgr

Because the entire point of the EU is to make transactions across member states as easy as domestic ones, and mandating support for 27 or so bank account formats in all contexts where they're collected sounds like a nightmare.

Also, how much longer are they really than legacy national standard ones? In the countries I'm familiar with, they've added exactly four characters: The country code in positions 1-2 and the robust, standardized check digits in position 3 and 4.

Both are extremely worth it, in my opinion. Take a look at ACH in the US if you want to see a great example for how not to do it: No checksums, two fields instead of one (ABA routing number and account number), separate routing numbers per domain (some banks have different numbers for wires, and some even for checks and ACH) etc.