First generation (i.e. 1940s vintage) radar used HF and VHF frequencies because high power amplifiers at frequencies above that simply did not exist. Such long wavelength radars simply cannot detect small objects no matter how much power was radiated. This is an over simplification, but think of the object being searched for as an antenna that re-radiates a portion of the RF energy that falls on it. For the antenna to work (and thus the object to be detected) at HF frequencies the object must be multiple meters in size. Not even the cannons on HMS Victory would show up on an HF radar. For VHF it needs to be around a meter or so. The cannons might show up, but essentially nothing else on the ship.
When magnetrons were invented (by the Brits!) kilowatt amplifiers in the microwave frequencies became available for the first time. The antenna/object can be as small as a millimeter or so at microwave frequencies. So radars operating in the microwave spectrum are able to detect much smaller objects, like all the metal bits and pieces on the HMS Victory.