If you're talking about using DNA to solve NP hard problems the reason is that it just kicked the problem to DNA quantity. While the computation was theoretical

If you're talking about using DNA to solve NP hard problems the reason is that it just kicked the problem to DNA quantity. While the computation was theoretically constant time, the DNA required would be exponential. If you make it suboptimal then may as well using an Ising computer. If you're talking about DNA storage, then cost, durability, speed, energy, reliability, size, etc. Are all issues.