Slow is not the opposite of haste, they are completely unrelated spells
Your problem is in assuming that these two spells are in any way related. They are not. Some of their effects may coincidentally be the inverse of each other, but that does not mean that they have any kind of underlying connection beyond the fact that they are more-or-less turning the same dial (action economy basically).
Slow, in fact does have an AC penalty which means the person is easier to hit. They do not suffer a penalty to attacks. RAW does not even say that the creature is actually moving in slow motion (though that is a common way to visualize the effects). Also, the actions of the creature are limited to an action or bonus action so there is indeed some restriction on the attacks that can be taken.
In the end, the spell does only what it says it does (and any additional uses/effects are up to your DM) and there is no secret part of the spell that you are missing. Only the designers can answer why they did or did not include things in the spell.
Other editions' rules are irrelevant
Every edition of D&D has rules that are completely independent of rules from the previous edition. It's better to consider them different games rules-wise. A penalty might have appeared in a previous edition and, if so, just means that the designers saw a reason to remove it. They don't have any bearing on our interpretation of the rules for the current edition though.
Sometimes it is better to accept that this is a game that doesn't always make perfect sense
The important thing to remember is that sometimes magic and the way the rules are written sometimes combine into situations that may not align with IRL common sense. In cases such as these you either just accept the incongruity as the necessary result of playing a game with a lot of moving parts or talk with your DM to see if it is an issue that they want to fix.
If you don't like the way the spell says it works get with your DM and change it. Keep in mind that making something make more sense does not mean that it will necessarily make that thing fun.