2 reasons :
1) As far as the benzene ring is concerned, concentrated
HNO3 will also give the same product. But Cambridge wants to test
if the student realizes that due to the 2 electron-donating by
resonance and thus strong activator phenolic OH groups
present, dilute HNO3 will suffice to substitute all 3 NO2 groups,
and concentrated HNO3 would therefore be an overkill (which
translates into a waste of resources and money, as well as
an unncessary additional safety hazard since concentrated HNO3
is highly corrosive).
2) The more concentrated the acid, the higher the risk that
the nearby C=C alkene group being reacted up, which is not
desirable in this question. Any acid (eg. HNO3, H2SO4, H3PO4) is
able to function as a catalyst to hydrate the alkene double bond,
to generate an alcohol.
As far as A levels are concerned, reason #1 is more relevant and
pertinent.