The lifetime allowance was cut from £1.8 million in 2010/2011 to a low of £1 million in 2016/2017 (now gradually rising again with CPI inflation) which added massive complications to the system with the need for various preservation schemes and affected a lot lot more people. At the same time the annual allowance was even more severely cut from £255,000 in 2010/2011 firstly to the current £50,000 in 2011/2012 and then to £40,000 in 2014/2015. This severe cut in the annual allowance on its own restricted the amounted of tax relief which could be obtained from then on hence there was little real need to cut the lifetime allowance as well - it was purely a tax grab by the government. The lifetime allowance made some sort of sense when the annual allowance was £255,000 (or even the £215,000 it was when first introduced on A-day in 2006) since without it severely high earners could gain really large amount of tax-relief for 40 years instead of which anyone making such large contributions would only be able to do so for about 7 or 8 years with the £1.8 million lifetime allowance.
For final salary DB schemes the restriction on the annual allowance also caused a lot more people to experience problems if after long service they received a fairly modest pay rise causing them to inadvertently exceeded the £40,000 limit. For higher earners the further tapering of the annual allowance limit also causes complications - and seems somewhat pointless as although these indviduals have high salaries they would without the tapering still be restricted to the £40,000 limit and hence restricted in the amount of tax relief they could receive.
In short the whole system has been turned into a complicated mess largely because the Conservative and Coalition governments saw it as a quick way to save money after the financial crash and pushed things too far. Leaving the lifetime allowance frozen at £1.8 million and making a more modest cut to say £60,000 (or at most £50,000) without introducing an annual allowance taper for high earners would have made for a much less complicated system and would have saved the government a reasonable amount of money but instead they introduced more drastic cuts and although it got them more money it also caused problems for a lot more people.