That's because Arthur Wellesley was busy at the time with the Peninsular War. Nobody else in England spoke Spanish, so they had to send him. He might have learned a thing or two about defense from Old Hickory at the Battle of New Orleans had he been there in 1815, but the Treaty of Ghent was signed and the war over before the battle. I must say that he did a bang-up job at Waterloo, however, with the Prussians arriving in time to take out Napoleon's right flank and, with some Anglo-Allied help, chase the French off the battlefield and force Napoleon's abdication and eventual exile. Whether or not the restoration of Louis XVIII was good thing is debatable.