Because its a synthetic element. The number that appears in brackets is the most stable isotope of technetium.
The standardized atomic weight of technetium is  amu.
why is the atomic mass of technitium usually in brackets?
Probably because technetium does not occur naturally; all its isotopes are radioactive with short half-lives compared with naturally occurring radioactive elements such as uranium.
because it is an isotope???
All isotopes of technetium are radioactive. Technetium is found only in trace quantities in nature, and then usually only in certain ores where it is a product of radioactive decay. All of the technetium used is synthetic.
Usually only in a laboratory; technetium is an artificial element that does not occur in nature.
Usually, brackets are used to organize a league's playoff games.
a low-level radioactive marker, usually technetium-99m or technetium pertechnetate.
Brackets are a way of ordering the priority of operations. In mathematics, usually things with brackets have their operations completed first.
Parentheses are usually placed inside brackets.
Usually when saying "periodic," scientists usually refer to the periodic table. This table is the shortened version of gasses, solids, liquids, and so on.