Pressure of developmental projects, inadequate staff with the ASI for protecting the monuments are the main factors. In Delhi alone, over 12 monuments are untraceable.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/around-3675-monuments-across-nation-untraceable
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/asian-age-new-delhi
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/monuments
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/urbanisation
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/archaeological-survey-india-asi
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/society-and-history
[7] http://admin.indiaenvironmentportal.org.in/category/thesaurus/india