Easy. What you wrote only works out if spells scale by caster level rather than slot level. This is an example of a 5e spell still pegged to scale by caster level using concentration and short duration to keep it from being too good when it does. The system used in 5e however is to scale by slot level. The upcast carries additional cost that the spell does not compensate for.How did you get that from what I wrote? I know that you can only have one concentration spell active at a time. My point is that both concentration and spell slots are limited resources. Spirit Guardians IS a more powerful spell than Bless - that's why it is 3rd level and Bless is 1st. But it's not a simple choice between one or the other. For the expenditure of a 1st level slot, a 3rd level slot, and Concentration, I can get Spirit Guardians and maybe Command, or Sanctuary. Alternatively I can also get Bless, and maybe something like Counterspell, or Revivify, or even Fireball if i'm a Light cleric. In addition, concentration spells can be interrupted early by damage and failed concentration checks. It;s less painful to lose a first-level spell prematurely after one round when you haven't gotten much benefit out of it, than it is to lose a 3rd level spell in the same circumstances.
Also, it's worth noting that most of the powerful concentration spells listed above aren't routinely available to PCs who can cast Bless, unless via domain spells. So in some cases, the choice is no choice at all.