Also, I'm convinced that if Signal were to become popular they'd probably sell it to some commercial provider, since the cost of maintaining a service used by hundreds of millions if not billions is extremely expensive.
WhatsApp is an entirely different matter. Hundreds of millions of Indians probably use it and if the company were to suspend service because the Indian government mandated a backdoor all hell would break loose!
For the same reason both Australia and the UK haven't as of yet forced WhatsApp to install a backdoor, even though legislation forcing them to do so is already in place there.