Spellcasters with misty step would become even more slippery
If a spellcaster ends up in melee range, misty step is a cheap, guaranteed option to get out without getting slapped by an attack of opportunity. It's basically a Dash and Disengage in one bonus action (and more, depending on what the teleportation lets you bypass). The only downsides are using a bonus action (if you even have any use for it), the spell slot (2nd level, pretty cheap), and the bonus-action spellcasting restriction.
With the restriction, a spellcaster has many options for what to do about an attacker in melee range, but they all have consequences. Disable/push them with a spell? Might come at a cost of using another spell on another target, and could fail if they make their save. Using misty step is a great option, but again may come at the cost of using an important spell. With the way it is, even a melee attacker that hasn't gotten a chance to attack the spellcaster yet (they dashed up or something) can have a significant impact.
Without the restriction, you could try to disable/push them with a leveled spell, fail, and then misty step away once you see it didn't work. Or you could go ahead and cast your more important spell on your more important target and still avoid taking any damage from the melee attacker.
It basically adds this layer of safety that changes the counterplay between spell casters and melee attackers, and reduces the impact of a melee attacker getting into range of a spellcaster.
Counterspell becomes less effective at stopping a spellcaster
Let's say you have an enemy spellcaster who has a spell you cannot afford to let them cast, and you have counterspell. In many scenarios, getting within 30 feet and not using your Reaction on anything else is all you need to do. As long as the enemy can't get out of sight and doesn't have more then 30 speed (more if you're closer), then they can't cast their spell without risking your counterspell.
However, without the bonus-action spellcasting restriction, suddenly that changes. Even if the enemy has 30 speed and is 5 feet away from you, they can just misty step for 30 feet, then move 30 more, and they're 65 feet away now, out of range of counterspell. You can counterspell the misty step, but then you've used your Reaction and can't counterspell the spell they wanted to use.
It destroys counterplay against spellcasters with devastating spells.