Because 3E broke grappling in half. It became too easy to "pin" an opponent in an inescapable trap that did round to round damage (or effectively paralyzed the opponent so everything could wail on them). 5E went the opposite extreme and really made grapple a very ineffective strategy.
When I design monsters that have grapple abilities, I usually build into them the ability to automatically do damage with a bonus action (ala a squeeze, bite or whatever was used to start the grapple). Rather than give them an "automatic grapple on a hit", I usually build/change them so that if they beat the target's AC by 4 they can start the grapple for free.
"Pinning" opponents is a real problem that mucks with the action economy and subverts the normal combat cycle. It's extremely efficient to give up your own actions to prevent a more powerful enemy from taking theirs, and its even worse if you, as the pinner, can continue to take your own actions while preventing another from taking theirs - its the main reason that spells such as Hold Person and Ghoul paralysis allow saves each round. Giving this ability, with no cooldown, to PCs is troublesome, especially in the face of Solo enemies. It's a little easier to give it a rare monster or two as they usually only show up for a single fight and it becomes part of the combat puzzle for that single combat. It could be an interesting addition to a dragon fight (with it pinning a PC under a claw), but gets unfun quickly if its showing up in every fight.
The same could be done for PCs, but it should be done with extreme care. Monk stunning is problematic enough, if PCs can use grapple to shut down an opponent they can and will do it. I did make a feat for my game that (among other things related to unarmed combat) gave the ability to use a bonus action to deal Strength mod damage to an opponent they were grappling. It's nowhere as good as what monsters can do, but its something.
I do think that creatures grappling each other ought to have a bonus to hit each other - maybe a +4 to hit or possibly advantage (allowing the rogue to sneak attack in a grapple). I'd be EXTREMELY cautious about preventing opportunity attacks or disadvantage to hit other targets as PCs can easily build into that to make it impossible for most opponents to hit from there.
