Can you name one manifesto that has been largely held to, especially on the 'big promises'?which is why it is so important to read the manifestos and make a decision on who you think is best to run the country well based on that.
And very soon most of those realise that the 'system' won't allow their ideological changes...Some do want to make positive changes, many put the country first.
So over time they simply fall into the 'if you can't beat 'em, join 'em' majority - self serving 'representatives' we have to endure!