Although the “Tax Cuts & Jobs Act” was billed as “Simplification” for the most part the opposite is true.
As far as reducing the return to post card size no one should be mailing their tax return on a post card because of identity theft. Decades ago it may have been a noble ideal to make filing your tax return so simple it could be filed on a post card. Today people E-File their tax return so this only amounts to a talking point for politicians. The tax law is certainly more complicated than decades ago.
The prior version of Form 1040 was a two page form. So while the new form 1040 has been reduced in size to be post card size most of what was on the prior version has been moved to six new accompanying schedules. It was far less confusing on the replaced two page form. You cannot mail seven pages as a post card anyway so little has been accomplished.