why cannot we use "me" instead of "I" in sentence 2 where "me" is used in sentence 1 ?- Sentence #1 is wrong. I have fixed it:
1) She is older than me. She is my elder.
2) He is taller than I. / He is taller than me. -- Both of these are in use. Some grammarians think 'I' is hyper-correct or falsely formal. Other grammarians