This is very common in the security field.

Good crypto/security software is often not very straight-forward for the public to find. They're usually run by competent developers doing the work for free and don't tend to have big PR budgets or do a lot of advertising. The product is findable, but they can be overshadowed by snake-oil products that focus on the business aspects.

Whenever a security project or service is attacked as insecure, the natural question is "what do you use instead?", particularly if the functionality was niche or unique. It's just a part of the community that an alternative good one is recommended. We try to avoid saying "don't use X" and instead say "don't use X, but do use Y".

Yes, it helps Y piggie-back a bit off the popularity of X, but that's Y's fault being being bad enough to be disparaged. It isn't just the TextSecure team doing this, it's everyone who cares about this type of service.