Why does Modi administration want SIM cards to be linked to Aadhaar card, especially when "mobile theft" is a big business in India ?
Aadhaar linking of sim cards is mainly directed towards monitoring of criminal activities and not mobile theft. In the past it was observed that terrorists and other criminals used sim cards registered in the name of Indian citizen (with fake or stolen address or identity). But now as Aadhaar number is unique and is a better identity proof (lesser chances for being fake), it is easier to track the sim card holder also the bio-metrics help in easier verification. So if a person's info has been misused for issuing fake sim cards the person will be notified and the card user can be tracked or the card can be blocked easily.
Earlier, if one needs to buy a sim, one needs 2 photocopies and 1 passport size photo. It was the biggest headache for government to identify how many sims hold by a one-person, People just buy sim cards whenever they want but after linking with an Aadhar card, Govt can track that how many sims held by one person.
Also, it's helpful to prevent serious crimes including terrorism. Agree with adithskv's answer.