well first off, the japanese did well in land battles in mainland china up until the end (when it got bowled over by crack russian units). so i am assuming you mean all the island campaigns fought mostly between american marines and army forces and ija forces.

there's a lot of reasons, but the one i want to highlight is equipment (including planes, tanks, ammo, etc) and by extension logistics. early on, the british were woefully undersupplied in malay and in singapore, and being so far from home, simply did not have enough tanks, ammo, fighter planes, and the like to fight a protracted war. actually the brits had totally cheaped out on their SEA army in the prior years. when the british fleet was devastated (losing wales and repulse), the japanese had total air superiority and supply chains (even though it was quite far from the home islands) while the brits had... pretty much nothing. the japanese were experienced at fighting combined arms with air support, mechanized units, while the brit forces had mostly warm bodies. pushed back to sg, they ran out of anti aircraft ammo and well couldn't hold out. (interesting to note that the japanese also were having supply issues at the end, but brits were way worse off and gave up despite having way more troops).

the reverse happened towards the end of the war. allied bombing and naval interdiction made it increasingly difficult to resupply the islands with troops let alone basic necessities. the us navy had by 44 completely dominated the waters and the ijn was quickly becoming irrelevant. thus the ija was left largely to fend for themselves while the us forces had complete naval supremacy (allowing them to bombard defensive positions and supply their forces at will) and air supremacy (allowing them close air support). the island battles towards the end saw a MASSIVE disparity in troop levels, as the us army could shuttle in whatever they wanted be it troops, ammo, tanks, etc, while the ija could barely feed itself. the ija really had no chance on any of the islands they controlled and were evicted from. as for the lopsided kill ratios, that has a lot to do with the japanese cultural stigma of surrendering. many troops effectively committed suicide (through ill-advised charges, etc).