Actually there are several reasons. One being that IH actually lead the way with inovations that it took other companies decades to adopt. More than 2 cylinder engines, Diesel powered farm tractors, row crop farming, shift on the go transmissions, hydrostatic transmissions, ceramic friction materials in clutches and etc. I can't verify that IH was first in all of these revolutions, but were responsible for many. And these only include tractors and do not include farm equipment designs.
Another reason can be found in the Nebraska test results showing time and time again how they out- preformed other similar hp sized models built during the same time period in torque rise and HP hr/gal figures.
As a mechanic that works on many brands of pre 1985 tractors, I can verify simplicity of designs with very few "Rube Goldberg" contraptions found in other "popular" brands. (Like the SCV, 24 volt split load electrical, pony motor starting and Permaclutch in JD).
The sheer numbers of IH tractors are still alive and well and farming around the world today in spite of the fact that the newest International tractors are now over a quarter of a century old, testify to their durabily and usablity.
Although I am an admittedly an IH fan, I will be the first to say that they and their products were not always THE best, but overall you would be hard pressed to (leaving opinons behind) honestly find another ag equipment producer that matched IH for innovation and contributions to designs and manufacturing proocesses that paved the road to our country being an agricultural giant that it is.
I don't have a family history involved in IH but I do have some good memories of using IH equipment. I worked on a farm as a teenager. The owner had at least three IH tractors, 300, 460 and another small utility tractor. I worked summers using all of them. He inherited all of them from his father and I helped rebuild one of them. They were aged but did a lot of work. The owner later bought newer tractors, but they all gave us problems. The IH never did.