Because everyday it is getting rarer.
Actually, approximately 3,600 brand new bitcoins come into existence every day.Therefore, unless there are more than 3,600 bitcoins being permanently lost per day, bitcoins are getting less rare everyday. Supply is increasing.
Correct, but the vast majority of us will be dead before the block reward drops to 0 satoshis somewhere around the year 2140, so bitcoin supply will continue increasing for our entire lives.
On the other hand, the block reward will get quite small in the next few decades. By the year 2048 the block reward will be 0.09375 BTC, so there will be approximately 13.5 new bitcoins created per day. If more than 13.5 bitcoins per day are permanently lost to the bitcoin economy, then the useable supply will become deflationary by then.
