The Tory succession has its own conundrums and questions of plausibility, both before and after voting for the candidates was introduced (1965). There's a question of at what point a 'surprise' PM/leader who suddenly emerged as a 'dark horse' candidate and beat the expected front-runners, such as John Major, emerged without really being noticed - and if they would ever have done so but for some unexpected events finishing off the expected winners. In Major's case, despite a reasonably good stint as Chief Secretary at the Treasury from 1987 he was reckoned as a short-term , unsuccessful Foreign Secretary in a brief period there in 1989 which could well have finished his prospects off. He was seen as only appointed as FS (to a job he had no experience for apart from a short period as a UK bank official in Nigeria, I think) as Mrs Thatcher's yes-man and as more malleable than Sir Geoffrey Howe, and after she reshuffled him to the Treasury he was just seen as a more malleable and less experienced candidate than the resigning Chancellor, Lawson. Yet within a year he was PM as the 'dark horse' winner as the 'stop Heseltine' candidate, though Home Sec Douglas Hurd was far better known and experienced (and was already an author and had been a 'backroom boy' aide to Ted Heath and a good N Ireland Sec). And if Heseltine had not stormed out of the Cabinet over Westland and thereafter been seen by party loyalists as a maverick , would he have been seen as too competent to be passed over in 1990 even if Mrs T distrusted him?
Was Major even so only 'plausible' in autumn 1990 as he happened to hold one of the top offices of state at the right moment, and as safer from Labour ridicule to floating voters as a self-made man from Brixton not an Old Etonian like Hurd? I seem to remember that earlier on the 'safe pair of hands' and 'rising young minister who can be trusted to do what the Men In Grey Suits tell him and not get out of hand' candidate to succeed Mrs T was seen as being (a) Cecil Parkinson - until the Sara Keays incident (b) John Moore (who he? - Health Secretary 1986) (c) Kenneth Baker - too easily sniped at by Spitting Image? And it is also noticeable who fulfilled the same role in terms of young, fairly competent, liked by Mrs T etc but wasn't seen as viable- ie Leon Brittan, Home and Trade Sec in mid-1980s . (Snobbery as he was Lithuanian by parentage?).
Earlier on, you can ask why someone who did succeed to the PM's office was not 'rumbled' earlier on, when there were doubts over him - eg Anthony Eden, a first-rated Foreign Sec and anti-appeaser (and seen as a victim of the appeasers esp Chamberlain) in 1935-8 and Churchill's wartime FS and deputy in 1940-5 and FS again in 1951-5 but fading by Churchill's resignation. Given his serious health crisis and subsequent dodgy health in 1953, which arguably affected his judgement over Suez, he was really a risk as PM as it was even then a straining job - and he had a reputation as being 'showy' but not solidly sensible. Churchill was said to have delayed retiring partly out of concern over his heir's potential as a disaster and to have said shortly before he did go 'I don't think Anthony can do it'. So if the next in line and most senior 'Mr Competence' , R A Butler, a capable party chairman and in 1951-5 Chancellor, had not been seen by the Churchillians as an appeaser (deputy For Sec to Lord Halifax and possible would-be negotiator with Hitler in 1940) would he have stood a chance of being chosen to succeed Churchill had Eden's health been better known or had it been leaked? Or was he too gentlemanly to stand in the 'rightful heir's way, as he was seen as doing in 1953 when both WSC and Eden were ill and he was acting PM, and as he did again in not standing up to Macmillan in 1957 or 1963?
When did Macmillan become plausible as Eden's replacement, and was this just as he was largely an unknown quantity and seen as a non-boat-rocker and a trustworthy former Churchill aide and minister (eg in N Africa 1943) and US ally who knew Eisenhower? And like Major because he was Chancellor when the PM went? Did his useful and publicity-minded stint as Housing Sec in 1951-4, building 'Homes for Heroes' in front of the TV, push him into the public consciousness - and if he had held a different, low-profile job (or Attlee had won the 1951 election and he had only just become a minister in 1955-6) would he have been seen as plausible at all? For that matter, even if Butler had botched the possibility of a 'Stop Home' movement by hesitation in 1963 as he did in OTL, if Macleod had not made so many enemies as Colonial Sec by quickly granting independence to African states (annoying the Tory right wing) and Hailsham had not been seen as too much of a showman could either of them have been able to stop Home, who was seemingly very implausible as a Wodehousian Scots aristocrat?
The 'surprise leader' question still hangs over the Tory party more recently - think of how William Hague, only Welsh Sec not of senior rank and aged ?36, managed to beat not only the distrusted Europhile 'front runner' Clarke (Chancellor and former Home Sec and Education Sec) but the more experienced Home Sec, Michael Howard, as leader in the 1997 party election. Was this again a vote for an 'unknown with majority views that back benchers will like' ' rather than a 'too clever and potential maverick outsider' , given Howard's immigrant parents and combative time in govt? And later on we have 'front runner' (but again of overseas origin) Michael Portillo not being chosen, and the obscure back bencher Iain Duncan Smith winning as the 'ordinary party members' grass-roots choice' despite not being the best supported man among MPs - the equivalent of Corbyn in the Labour Party?