There was no rule, but it was basically the only convenient way. Receiving e-mail on a phone was not at all common, typing a long URL on phone was a PITA and paying for stuff online was not something a lot of people were familiar with.

WIndows CE phones and the like were so niche there was no point in even developing apps specifically for them.

Also note that the above would usually only work in one country, if you wanted to sell internationally you’d have to make arrangements for a shortcode and RB-SMS for each country you wanted to sell in. Never mind the advertising campaigns. Apple taking care of that, with basically global reach and different kinds of payment methods without you having to worry about any of it was quite revolutionary.