Ignoring all the usual India hating, I will try to answer this question objectively.
- Long coastline does not necessarily mean there are beaches all over. Coastline can be rocky (as in European countries) or located in very hot and dry areas (like Middle East, and parts of Africa and Pakistan). These don't serve as good places to hang out. Gujarat coastline is pretty useless. Tamil Nadu, Andhra Pradesh and Orissa are very hot, Bengal is mostly a delta. That leaves only part of the western coast.
- Beach spots are made 'famous' by international tourists. The vast majority of tourists thronging beaches in Thailand, Maldives or the French Riviera are not locals but foreigners. These people want infrastructure along with the beach - a nightlife (music, widely available alcohol, beach beds), water sports, bikes on rental to move around cheaply etc. Apart from Goa, and to some extent Kerala, Indian beaches don't provide this.
- Cleanliness is an issue. The famous beaches in India are not dump yards as some PDF members would like the world to believe, but there is a general consensus that the water is not as clean as in other famous beach destinations. Industrial pollution is a big culprit. Most beaches are clean, but water is not.
- People - Pesky touts and constant attempts to over charge result in a bad taste in the mouth. This is not a beach specific problem, but a problem in all tourist areas in India. A huge negative.
- Andamans are much better, but not marketed enough. Besides, tourism infrastructure is not well developed. For too long it has been treated simply as a remote Indian outpost. It has piss poor connectivity. No government has had any vision regarding this region, because it is not politically important as a domestic constituency.
- Lakshadweep are great too, but too small and fragile. The locals themselves don't support much tourism activity. Foreigners get the same type of beaches in Maldives, plus a better assortment of resorts from international brands, which means more competition and therefore better standards.
- People (again) - You need the local population to be chilled out about beach tourists. Much of India is conservative when it comes to clothing. Women in bikinis still draw unwanted attention. Foreign tourists want to move around freely in towns without feeling the need to change clothes every time they visit a local attraction
- You don't get very good value for money. India has a large population, which drives up real estate cost and thus rentals. Properties in busy areas have high running costs mainly due to high rentals. This translates into higher tariffs for rooms and F&B. In busy seasons one has to struggle to get a good hotel room on a budget.
I have been to multiple beaches in Thailand and they are not spectacularly better than Indian beaches. But yes, they perform better in all the areas I have mentioned above. I have been to Maldives too. It is an entirely different tourism market, not for the regular joe. Expensive and you are stuck to your island. Good for honeymoon or anniversaries but as a destination very different from the rest of the world.
