There are quite a few reasons for it -
In a blockchain implementation, every block is verified independently multiple times before it is added to the blockchain. On the other hand, many git projects do not require independent verification and, when they do, they only require one person to sign off on a change before it is committed to the repository. Hence, with at most one point of validation that you must trust, git breaks one of the core tenets of blockchain technology.
Secondly, git history can be re-written which negates a very important concept of Blockchain i.e. the immutability of the ledgers.
Thirdly, A git repository is not necessarily duplicated on many servers. You can work from a gitrepository locally and if your local disk were corrupted, you would lose everything. Blockchain technology implies the reproduction of the ledger across servers.
Blockchain is unlike Git in a way that everyone strives to work on a single branch. In Git everyone may have several branches and fork and merge them all day long. In Blockchain one cannot “merge” forks. Blockchain is a actually a tree of transaction histories, but there is always one biggest branch and some accidental mini-branches that have no value at all. In Git content matters (regardless of the branch), in Bitcoin consensus matters (regardless of the content).