The best selling does not mean it is best. It sells more because it is an average car priced lower in its category. Most of the middle class people of the world are cost conscious. Corolla is the highest selling car, does that mean that it is better than lower selling cars like accord, camry, mark x etc ? No.
Immobilizer is a very strong barrier for theft. Its not impossible to crack it, but it is very hard and it takes a long time. Time is the thing the theives thrive upon. The more time it takes, the more difficult it gets to steal a car.
And the immobilizer and passenger airbag is present in the Corollas in most of the countries. Indus motors have an issue providing these features.