It's specific to a subgroup of enemies, which you need to be able to accurately identify in game. And the concentration is a huge downside. It needs to rank higher, more often, than other concentration spells. And ten minutes isn't particularly valuable in a concentration spell for a melee front-liner.
Counter-counterpoint: it covers a huge chunk of the more dangerous enemies. Not having protection against humanoids, giants, and dragons does hurt -- but not having protection against constructs and animals is not a big deal. And except for dragons, having another Disadvantage-On-Demand effect (such as Blur or Greater Invis) will cover you. The other thing is that blindsight and truesight and other such senses completely foils the standard 'disadvantage on demand' spells: Mirror Image, Greater Invisibility, and Blur. The only relatively cheap way to force disadvantage for a long time against a large group of enemies is Protection From Evil.
I also think you're undercounting the value of a 10-minute concentration spell. I've played in plenty of scenarios both in 5E and other D&D games where you anticipate combat, buff up and charge in, but the trouble doesn't arrive immediately because you misjudged the shape of the room / need to investigate something / have talky-time first / etc. These things taking a minute of delay will kill most debuffs, but 10 minutes makes it a lot more likely to have your buff going in.
And if you completely misjudge the timing, so what? It's a 1st-level spell. It costs 25 gp a pop, but Bladesingers are really vulnerable to getting ganked in the first round of combat, even with shield. If you have a better Disadvantage-On-Demand effect to cast, then just drop concentration and cast it. You're out a 1st-level spell slot, which stings since they compete with your Shield and Silent Image slots, but better than going into combat with only a 16/17 AC.
All that would be good enough, but you also get Charm and Frightened protection to boot.
Protection From Evil:
Pros: First-level. Lasts 10 minutes, making it easier to head into combat with it. Provides Charm and Frightened Protection.
Cons: Concentration. Does not protect against Humanoids, Animals, Constructs, and Dragons -- and the first two are very common enemies at low level. Costs 25 gp a pop, making it prohibitively expensive at low levels.
Blur:
Pros: 2nd-level. Gives blanket disadvantage against everything that doesn't have special vision.
Cons: Concentration. Completely foiled by special vision or effects like Faerie Fire.
Mirror Image:
Pros: 2nd-level. Does not require concentration. Provides perfect defense against AC attacks
Cons: Enemy still has a base chance to hit you. Mirror Image AC is a lot lower than your AC, so you will lose images when you otherwise wouldn't. Limited protection, which means that it's hard to stand up against hordes just with this spell.
Blink:
Pros: Makes you completely untargetable by not being there. Non-concentration.
Cons: The blinking effect is random. You also lose the chance to provide melee interdiction while you're gone.
Greater Invisibility:
Pros: Makes you untargetable by spells. Makes it so that an enemy has to outright guess where your square is. Gives advantage. Gives a huge bonus to stealthing.
Cons: Concentration. Higher-level slot. Completely foiled by special vision or effects like Faerie Fire.
HONORABLE MENTION:
Haste, its extra action used to hide in combat.
Pros: Good use of your concentration slot, since Haste has a lot of side benefits. Hiding gives great defense as long as you're not noticed.
Cons: Being able to hide is really hard to do in a lot of circumstances. Having some way to drop out of enemy's sight (Cunning Action, Bonus Action Teleport, magic items, being a Lightfoot Halfling, etc.) is mandatory. Will auto-fail in a lot of circumstances.
Nadrigol said:
Using an ASI and your RA for some off-turn damage doesn't seem like enough. Plus if you're carrying a Quarterstaff you have one less sword whether you're single wielding or dual wielding. I'm not seeing the appeal.
Booming Blade is just the Quick and Dirty, No Resources use for the spell. Unless the PHB got errata, you can use Warcaster with any spell IRRC. Use it to set up Melf's Minute Meteors or Blindness for example.