Indian here. Simple answer is that there just isn't anyone left to stand against his behemoth of a party (BJP).
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.