I'd love to hear somebody from India explain it: my guess is that religious politics and political religion have been India's problem from before the partition, and it's a fissure that Congress typically tried to ignore, while, rather awkwardly, it's been sticking its elephantine trunk in everybody's drinks while taking up eight tenths of the room.
Modi drives straight at the problem, takes a position, then relies on the demographics to carry him through. There are more Hindus, so in a democracy, taking a Hindu-first perspective is sound electoral strategy.
The Congress/UPA's reputation was severely trampled out when BJP/NDA rose to power in 2014, and it hasn't recovered since. BJP has perfected the art of media manipulation and public politics, and is extremely competent in directing and subverting public sentiment; they are one of the biggest populist forces in the world right now with a formidable and highly-skilled marketing force backing its brand (enough to get featured on Harvard whitepapers). More importantly, they learn and adapt quickly to changing environments. For example, after the JNU fiasco they now seem to recognise the importance of moderation and avoid going gung-ho on dissent in practice (e.g. their prolonged tolerance to the Farm Law protests) while keeping a public image of firebrand rightism to maintain their brand. They are intelligent, very intelligent.
Its opposition, meanwhile, has only gone downhill and keeps embarrassing itself. Opposition parties neither have a well-built plan or timeline, nor do they invoke any significant loyalty from voters. They keep infighting and bickering among themselves while retaining dynastic figureheads from an already disgraced family. They're all so in on themselves they still try to use the same half-baked strategies that used to work in the 2000s.
Do you know why third parties haven't come to the fore? I can see that the Congress party has a bunch of problems that are pretty inherent to the party DNA, but did the people who used to vote Congress because they liked secularism and universalism just stop liking those things?
BJP/NDA simply outclasses every other force in terms of its outreach and marketing prowess, and with the kind of control it now has over mainstream media, challenging them will be a herculean task at least for the time being (and filled with a very tangible risk of self-destruction).
At the end if the majority (mostly boomers) that do not want change continue to do this it is saddening for the young. But of course, when it bites them it may be too late.
Sad but democracy in function.
Especially the UK, who have been trying to prop up the Gandhi-Nehru family party, their heirs and Brown sahibs.