While this case may have been a very tricky but why some are able to respond to treatment while others struggle a lot. It may be a rhetorical question for now because medical science has not reached that far.
One thing you will soon see from reading this forum is that no two Crohn's cases are exactly the same. IMO, there is no one definitive disease that defines Crohn's. Instead, Crohn's exists as a spectrum of similar, overlapping conditions that share many (but not all) features in common that may be triggered and/or worsened by different factors. And this variability results in a wide range of disease severities.
With this in mind, it becomes easier to see why some respond to treatment and others do not. Some just got dealt a worse hand of genetic susceptibilities and environmental triggers, resulting in disease manifestations that range from so mild as not needing any treatment to catastrophic Crohn's that is impervious to pretty much all therapies.
It's worth noting that prednisone, azathioprine, methotrexate, Rinvoq, Humira, Stelara, and Entyvio all, to one degree or another, dampen the activity of immune system, resulting in reduced disease - hopefully to the point of achieving remission. But all seven of these drugs do it by different biochemical mechanisms. They attack the complex network that comprises the human immune system at different. points, all with the same goal - to walk the knife edge of tamping down the immune system sufficiently to relieve the Crohn's inflammation but without falling off the other side and leaving the patient vulnerable to every infectious microbe that comes along.
This multi-pronged approach works reasonably well for most patients, and with enough trial and error some drug or combination of drugs usually can be found, sometimes aided by dietary changes, that gets the patient into some reasonable form of remission. But every now and then this approach is insufficient; a particular version of the disease comes along that is simply too severe for any of these drugs to work very well.
