Why the blockchain isn't just a Merkle tree I am reading this and now wondering why the blockchain isn't just implemented as a Merkle tree, or the reverse, why you can't just have a single data structure (i.e. a transaction chain) instead of the block chain with nested Merkle tree of transactions.
A bitcoin full node independently validates the history of the network. To do so, it must validate each block in sequence (which includes all transactions in each block). 

A merkle tree provides a cryptographic commitment to some data, but does not hold that data itself (it just holds hashes of the data). So a merkle tree alone is insufficient to validate the history of the network. Nodes need block and transaction data to do so, not just the merkle tree commitment.