Why do you think that grammar is slower in change?
Well, Bianca I am of course not sure but I would say:
1. Lexis is more dominat in everyday life than grammar. Meaning is mostly made with the help of vocabulary not grammar. The lexical approach puts more emphasis on grammaticalised lexis (see the following URL)
BBC | British Council teaching English - Methodology - Lexical approach 1
2. lexis can even replace grammar particularly the tenses. For example instead of using the
going to form you can say we
plan to, intend to.
3. When a language is learnt as a foreign language or when children start speaking it is lexis not grammar which dominates. But again words don't follow one another at random. There is always some kind of stringing or threading sth on sth: collocations (grammaticalisation of lexis). The two areas of language are so closely intertwined that neither of them can exist independently.
Jamshid