1987: splits and turmoil in Labour Party, with SDP defections and some rebellions by the 'Militant Tendency'. Kinnock was more a compromise candidate than a cause of the party splits. Kinnock was portrayed by the media as soft on the Soviet Union. Most of all, the Tories bribed the electorate with Council House sales and sale of shares in privatized utilities.
1992: Extreme Tory bias by much of media, even more than usual. As the Sun claimed, 'It was the Sun wot won it'. Over-confidence by Labour; triumphalist Sheffield rally that probably put some voters off. Major was less personally off-putting than the by-then very controversial Thatcher, and was not so directly linked with the Poll Tax.
It's true that Kinnock was not a very effective leader, and that he was regarded by some as a 'windbag'. But I don't think that the main reason for Labour defeats was his being regarded as insufficiently principled or left-wing. If that were so, Blair would hardly have won his elections (I am just the messenger here; personally I voted for Kinnock and did not vote for Blair). Moreover, voter turnout was high in both the 1987 and 1992 elections, suggesting that there was not a high proportion of people staying at home because they had no one to vote for. Voter turnout only started to decrease dramatically in 2001.