As stated by the source, year-to-year comparability of the "all returns" total is affected by changes in dollar income filing thresholds, while year-to-year comparability of the number of returns by type of tax form used is affected by changes in the specific filing requirements, including adjustments for inflation, for each form.
In general, the filing of "short forms" (mostly Forms 1040A and 1040EZ) has been encouraged by IRS to simplify both taxpayer filing requirements and IRS processing. Filing requirements for each of the short forms have, therefore, been gradually liberalized to facilitate their use. Increases in the number of short forms have mostly been at the expense of "long form" 1040 usage (although increases in the number of each of the short forms have often been at the expense of one short form over another). While the number of 1040 (paper) returns has, therefore, increased over the time period shown, the rate of increase has been reduced by the use of the short forms, and, later, by electronic filing options.
For additional information about the return filing requirements, see the annual reports, Statistics of Income-Individual Income Tax Returns at https://www.irs.gov/uac/soi-tax-stats-historical-data-tables.
Forms 1040, 1040A, and 1040EZ include "standard" electronically-filed ("ELF" or "e-file") returns, and Form 1040EZ returns filed electronically by telephone ("TeleFile" for 1999-2004 only). Form 1040PC (computer-generated "paper") returns were not reclassified for Statistics of Income for 1999; data for Forms 1040, 1040A, and 1040EZ for this year are not comparable with preceding years (Form 1040PC did not apply after 1999). For 2006, the total number of Form 1040EZ returns includes 742,859 Form 1040EZ-T returns (the only year it existed).
U.S. Department of the Treasury. Internal Revenue Service, Individual Income Tax Filing: Form 1040 Returns [RT1040FRM], retrieved from FRED, Federal Reserve Bank of St. Louis; https://fred.stlouisfed.org/series/RT1040FRM, May 20, 2019.