Because there is no single absolute "best" — what counts as best depends on which criterion you optimise for, so "best" is always relative to a chosen yardstick.

The scare quotes are a warning against treating "best" as an objective property of a solution. A solution can be best on one axis and poor on another:

• Fastest is not the same as most durable.
• Cheapest is not the same as most valuable.
• Easiest to build is not the same as the one users gain most from.

So before declaring anything "best", you have to ask best for what, and for whom? Naming the criterion turns a vague argument ("this is obviously the best option") into a defensible one ("this is the best option given that we value speed over polish here").

Tip: Whenever someone insists a solution is simply "the best", ask "by which measure?" — the quotation marks are doing exactly that work.