Why do so many people favour Tru-oil?

I think the answer to that is, It gives a pleasing appearance and applied correctly, to the manufacturers instructions it is easy to use.

Why do people have problems with it?

Because they apply it too thick or perhaps more likely, they are too thick.

Why do people use Pro-custom, Permalyn, & Tung oil?

Because they are looking for an alternative and perhaps have had problems with other types of oil finish.

So it is, as always left to personal preference.

I think I have posted the following comment before.

I was finishing a stock for an Italian gunmaker and we were discussing finishes, and he said that the oil finish for the American market that was applied by him as standard was Tru-oil.
'Best' quality was traditional oil finish.

I have one customer who always specifies Tru-oil and another who prefers French polish.