Because that is the way the bitcoin protocol was designed to work.  Every 210000 blocks, the block reward of freshly minted currency that the miner receives is reduced to half the value of what the previous block reward was.  This is the reason that there will never be more than 21,000,000 bitcoin in existence.

The total supply of bitcoin will continue to increase, it will just increase slower than before. This will only result in an increase in value if an increase in demand outpaces the increase in supply.  It is possible that the value will increase in the future, but it is also possible that the value has already increased in expectation of the reduction in block reward and that a slower increase in demand causes the value to drop.  The future can be difficult to predict reliably.