Very likely the reason Remington introduced the twin action bars in the Model 870.
Also the way the barrel mounts into the receiver isn't as rigid or as precisely aligned as is the M31's in relation to the bolt's lock-up cycle/operation. The 870 needs twin bars for smooth operation. The 31 was an offspring of the M17 with a better barrel to receiver connection than the 870 but more expensive to make. The 870 is the AK47 of pump shotguns. Stamped parts that work better than just pretty good and as reliable as any ever made--the old Wingmaster but the cheaper made Express has quality control issues.
