The choice of a lower working pressure in Instant Pot is a trade-off of function and cost. The pressure cooker industry safety standard ANSI/UL-136 has a stress test case which requires no leaking at 5 times the working pressure. For 15psi cookers, this is 75psi; for 11.6psi Instant Pot, this is 58psi. The differences in material and construction are huge. We could build an Instant Pot that operates at 15psi, but not at under £100-150 level. It would be more like £300~500 level and our research indicates that most people are not willing to buy in that price range.
On the other hand, operating at 10.15~11.6 psi only results in 7~15% increase in cooking time, i.e. about 3 more minutes in a 30 minute cooking. With the set-and-forget convenience, the extra cooking is more than tolerable. Furthermore, thanks to the air-insulated housing, maintaining pressure for the extra time consumes very little electricity. Our study shows that Instant Pot uses less than half of electricity comparing with a stove-top on a electric range, taking the same length of time.