Its because tgey take to long to get good relative to other classes.
If it was a higher tier game they would be A or S subclass and level depending . Theres better spellcasters with more resources. Twin spell for example.
If youre 5MWD say level 5 they're also better. Also DM and campaign...
5 or 6 attacks a round. Level 14 proficiency in all saves.
At close to those levels a lot of saves are creeping up around 17-20 DC.
Lots of fear effects and other ruin your day abilities eg hold spells.
Ranger falls off around 10, Barbarians a one trick pony. Paladins and Fighters also...
Keeping an eye on this thread to mine for new campaign. Already going to mine PotA and RotFM for some dungeons to stick in Anauroch. Might have to buy Storm Kings Thunder or see if one of my players has it.
OP stuff like twilight clerics and crap like the berzerker.
Find a tier list and aim for B+ to A levels imho. Older dungeon dudes subclass tier lists are a decent place to start.