its because a new block is made every 10 minutes as a record of all transactions that happened in that past time.
since i'm terrible at explaining, heres a section from the bitcoin wiki on blocks:
Each block contains, among other things, in its block header a record of some or all recent transactions, and a reference to the block that came immediately before it. It also contains an answer to a difficult-to-solve mathematical puzzle - the answer to which is unique to each block. New blocks can't be submitted to the network without the correct answer - the process of "Mining" is essentially the process of competing to be the next to find the answer that "solves" the current block. The mathematical problem in each block is difficult to solve, but once a valid solution is found, it is very easy for the rest of the network to confirm that the solution is correct. There are multiple valid solutions for any given block - only one of the solutions needs to be found for the block to be solved.
Because there is a reward of brand new Bitcoins for solving each block, every block also contains a record of which Bitcoin address is entitled to receive the reward. This record is known as a generation transaction, or a coinbase transaction, and is always the first transaction appearing in every block. The number of Bitcoins generated per block starts at 50 and is halved every 210,000 blocks (about four years).
Bitcoin transactions are broadcast to the network by the sender, and all peers trying to solve blocks collect the transaction records and add them to the block they're working to solve.
The difficulty of the mathematical problem is automatically adjusted by the network, such that it targets a goal of solving an average of 6 blocks per hour. Every 2016 blocks (about two weeks), all Bitcoin clients compare the actual number created with this goal and modify the target by the percentage that it varied. This increases (or decreases) the difficulty of generating blocks.
Because each block contains a reference to the prior block, the collection of all blocks in existence can be said to form a chain. However, it's possible for the chain to have temporary splits - for example, if two miners arrive at two different valid solutions for the same block at the same time, unbeknownst to one another. The peer-to-peer network is designed to resolve these splits within a short period of time, so that only one branch of the chain survives.
The client accepts the 'longest' chain of blocks as valid. The 'length' of the entire block chain refers to the chain with the most combined difficulty, not the one with the most blocks. This prevents someone from forking the chain and creating a large number of low-difficulty blocks, and having it accepted by the network as 'longest'.
https://en.bitcoin.it/wiki/Blocks 
