I guess as a starting point we should quantify "few" - assessing aerial victory claims is a notoriously fraught process (see e.g. How did the military verify if a pilot did shoot down another enemy plane during WW2?), so completely definitive lists are all but impossible to assemble, but using the main nationalities from Allan Magnus' Air Aces Homepage as a guide it lists around 744 RAF aces, comparable to the 717 USAAF aces but considerably less than the 2489 of the VVS (Soviet Air Force) and 2864 of the Luftwaffe (figures approximate due to the structures of the lists; Magnus also notes "According to some references, the German aces of the Luftwaffe number over 5,000. I personally doubt the veracity of this"). Adjusting criteria to nationality slightly changes the picture - the RAF included Commonwealth and other foreign squadrons, 503 of its aces were British, whereas the US Marine Corps and US Navy add a further 120 and 374 aces respectively to the US total - but certainly there were fewer aces from the Western Allies. In addition to overall numbers, the claims per individual is considerably lower - over 100 German pilots have more than 100 victories, the highest Allied total is 62 for Ivan Kozhedub, only a handful of British or American pilots have more than 30.

There are a number of reasons for the disparity. First of all, training. The RAF rather neglected gunnery before the war; it envisaged battles between unescorted bombers and fighters being more common than dogfights, with a consequent emphasis on formation flying, aerial gunnery was something of an afterthought. This compounded the second factor - experience. It took time for pilots to become used to aerial combat, their first few combat sorties were usually a confused mess, and it took time for air forces as a whole to adapt their tactics. The German Air Force had a core of veterans from the Condor Legion of the Spanish Civil War, where they had developed the highly effective Schwarm of four aircraft divided into two Rotte, and gained further experience over Poland and France. Though the RAF had deployed some squadrons to France many of its pilots were experiencing combat for the first time during the Battle of Britain (some straight from training), and at the beginning of the Battle were using the Vic formation of three aircraft, a much less flexible or effective formation. Nevertheless, Fighter Command shot down almost twice as many aircraft as it lost in the Battle of Britain, a large proportion of them German bombers (though if you factor in Bomber and Coastal Command losses in the same period there's less of a disparity; see Did Britain win the battle of Britain? for more).

After the Battle of Britain the key factor is really opportunity. Following the German invasion of the Soviet Union much of its air force strength, especially bombers, was on the Eastern Front; attacks on the UK were in greatly reduced strength, either by night or high speed 'tip and run' raiders. The Soviet Air Force was large, but (at least initially) not of the highest quality; the majority of the claims of the highest scoring German aces are from the Eastern Front. Fighter Command attempted to draw the German Air Force into battle over northern France but the limited range of the Spitfire and Hurricane meant this was ineffective; meanwhile there were still concerns over a possible resumption of large scale aerial attacks against Britain or even an invasion attempt so the most modern aircraft were retained in the UK while older, generally less effective, types were deployed overseas to e.g. North Africa and the Mediterranean (no Spitfires were deployed overseas until 1942). There were a few 'hot spots', such as the island of Malta that was subject to heavy German and Italian bombing, but of small scale. As the Allied Combined Bomber Offensive gained momentum this became the main aerial battleground in the west; German night fighters engaging Bomber Command at night, and German fighters engaging the USAAF by day. Initially there were no Allied fighters involved, but with the USAAF taking unsustainable losses on unescorted raids the long-range P-38, P-47 and particularly P-51 were employed to sweep the skies over Germany, giving American pilots the opportunity to engage the Luftwaffe. The RAF showed no interest in developing similar long-range escorts - John Terraine, in The Right of the Line, puts the blame firmly on the Chief of the Air Staff (Charles Portal) who was convinced that a long-range fighter would never have the performance to match a short-range interceptor and thus opposed any such developments. James Holland in Big Week also points the finger at Sholto Douglas and Trafford Leigh-Mallory, AOCs Fighter Command in 1942-43, for a lack of will and imagination. An RAF pilot based in the UK might not see an enemy aircraft in a tour - e.g. Roland ‘Bee’ Beamont in an interview with James Holland:

"In the years between 41and 44 you’d very seldom see enemy aeroplanes because they wouldn’t come up for small formations. They’d only come up for big formations. I did a tour on Typhoons from 1942 to 43 in the middle of the war. The emphasis on that tour was all overseas. It was all ground attack. [...] We never saw anyone. I must have done more than 100 low level sorties in Typhoons starting in November 42 and finishing in May 43 and I never saw any enemy aeroplane over France. And that was the experience of most of the pilots."

The idea of a tour is another key difference between the RAF and USAAF and the Luftwaffe. In the former pilots completed tours of a certain number or sorties or certain number of hours (depending on where they were stationed and the phase of the war) and were rotated to other duties - often training new pilots at Operations Conversion Units but also e.g. aircraft testing or liaison work - before another tour. German pilots tended to fly, and keep flying; Mike Spick notes in Allied Fighter Aces that Erich Hartmann, the leading ace of the war, flew 1,352 sorties and encountered the enemy on 825 occasions, whereas few Allied pilots flew more than 400 sorties, and would have encountered far fewer enemy aircraft.

As a final note, the RAF also had a more collective culture, with less emphasis on the individual. Personalities naturally varied, but in general boastfulness ('shooting a line' in RAF parlance) was heavily frowned upon. The September 1942 edition of Flying and Popular Aviation magazine devoted to the RAF, for example, has an article on The Men In Action:

"The men of the RAF are largely inarticulate; they would prefer anonymity; and perhaps the worst of all sins in the RAF is the process known as "shooting a line". [...] To shoot a line is now embarrassing, boring, comic, or in plain bad taste. In this way the RAF has elected to speak even of its most exceptional achievements in terms of understatement."

James Holland and Stephen Bungay, writing about the Battle of Britain, both contrast the British and German attitudes towards individual victories and decorations:

"There was ambition in the RAF too, but striving to improve personal scores was deeply frowned upon. [...] Any kind of boasting was deplored, and anyone who did was immediately accused of 'shooting a line'. Unofficially, a pilot became an 'ace' when he had five confirmed kills. It was an important marker and usually earned a pilot the Distinguished Flying Cross, but a DFC could be earned without a pilot having reached five kills. Certainly there was no cult of the individual." James Holland, The Battle of Britain

"The RAF, in contrast, refused to officially recognise aces and only reluctantly co-operated with press interest in the likes of Tuck, Malan or Bader. [...] In the RAF, DFCs tended to be awarded for 'distinguished service', but the criteria were less explicit. The highest decorations, including the VC, were input-driven [as opposed to the 'output-driven' German system based on victories]. The VC was 'for valor', in other words, extraordinary courage, whether or not it was militarily effective." Stephen Bungay, Most Dangerous Enemy

Further reading & watching:
Allied Fighter Aces of World War II, Mike Spick
Aces High, Christopher Shores and Clive Williams
Why did the German Aces have so many Air Kills?, Military History Visualized