With real time, you lose a lot of nuance of the 5e rules, as a cleric you have to take more than 1 round to be able to move and cast a heal on a party member for example with real time, when the rules actually allow you to move and cast in your turn. In my experience with BG the enemy will still be attacking when I could have had the time to move and heal in a turn based environment.

Now I understand that people just want their classic BG experience, and that is fine, I'm sure they could do both with their engine, but If it is really based on 5e, it would be a shame to miss out on a lot of the combat rules. If you need to keep pausing to take advantage of the rules or to line up your magic, why not just have it turn based anyway?

You can move and cast heal in the same round. BG1 and BG2 still track turns, so within one turn you can cast one spell and until the end of the turn you won't be able to cast another one, but you can move as much as you want before end of the turn.

While some classes have interesting things to do every turn, like clerics, druids or sorcerers, there are classes that are less engaging, for example Warlock often keeps casting Eldritch Blast until his target is dead, because they have very limited amount of spells. The same with warrior, often for the most of combat he just keeps hitting his target with a sword until the target is dead. It's not fun to keep doing it over and over and over again in a video game, so I like that in BG you just order a warrior to attack an enemy and he will keep hitting it until it dies or you change that order. No need for micromanagement of each basic attack.

Also BG1 and BG2 is known for combat encounters with many opponents. Like storming Gnoll Fortress and murdering dozens of Gnolls in one battle. On top of that you have 6 companions and you can summon monsters, skeletons etc. All that leads to situation where encounters with 30+ combatants are common. If I had to wait for all 20 gnolls to do their turn and then wait for my summoned pack of dogs to finish their turn before I order my warrior to hit a gnoll with a sword and then my warlock to cast Eldrich Blast etc. and then wait again for all those freaking gnolls to do their turn again, then I would never finish BG1.

One could say that the solution is to make encounters small, so you don't have to wait for 20 gnolls/gibberlings/kobolds to finish their turn first, but I would say "OVER MY DEAD BODY!" It was great to slash your way through army of gnolls after you leveled up enough. Heroic fantasy at its best. Combat was intense, fast and yet still tactical. I have many great memories about making the last stand in some choke point against overwhelming number of enemies and I hope to make new ones in BG3.

Also enemies in BG don't scale up, so if I visit a lower level location with my experienced party, I still might encounter a pack of kobolds. In RTwP it takes to 10 seconds to clear them and continue my adventure. In turn based I would have to wait for each of them to do their turn and I would waste 5 minutes for a combat where my enemy don't even have a chance to really hit me.