For what reason cannot taxpayers amend their return to claim credits for previous years?

The correct answer is rooted in the procedural requirements for claiming tax credits and the importance of having all necessary identifying information by the due date of the original return. Taxpayers cannot amend their returns to claim certain credits if they did not obtain a required identification number, such as a Social Security Number or an Individual Taxpayer Identification Number, by the due date of the original return. This requirement ensures that all taxpayers are properly identified and that the credits can be accurately attributed to the correct individuals.

When the identification number is missing by the due date, it can hinder the IRS's ability to process the return and can invalidate claims for credits associated with that return. Thus, if a taxpayer failed to secure the necessary identification number in time, they’d be ineligible to amend their return to include those credits for the prior years, maintaining the integrity of tax administration.

The context of the other options highlights various other rules and restrictions concerning tax returns, but they do not directly address the critical requirement of having an identification number in order to claim credits successfully.