While Raku's answer is correct - these decisions/implement details don't concern your clients/customers, you don't seem to appreciate the business factors facing your customers.
It's likely they understand the importance of maintenance, but other factors take priority.
Another answer on another question highlighted the time to market factor for mobile games. It is better for business to have software that to sell today than tomorrow, especially if competition is developing something similar.
It is better for business to have usable software in the wild than something sitting in the lab being re-tested and re-tested.
Your customers will decide whether the investment is worth it and if their product is successful they can reinvest in maintainability, but based on those decisions you have to toe the line to get it done for the cost/budget quoted (if you use TDD, then time for your development is factored in estimates for these quotes). If you've got time to do TDD within that quote and your estimates, you should use TDD, but if you don't have time then you can't afford to do it.