Because even strong software patents often have alternatives, and weak ones are easy to find an alternative to, decreasing their value, but once a standard protocol requires their use, the patent holder can jack up the price. As an extreme example, consider the exFAT patents: there are many file systems superior to exFAT that have no need for whatever bits of functionality Microsoft patented, but because Windows speaks exFAT as one of few alternatives, some companies have licensed them. Not exactly a standard, but the same idea.