mining difficulty

[loicatraile]

hi guys!
 i was just wondering why is that is getting more difficult to get a profitiable coin mined when for example the difficulty for mining that coin stays the same?.. for example if you look at www.coinwarz.com ..a few weeks ago to get 1 btc in dogecoins you would have spend aprox. 30 days mining with a hash rate of 1000 kh/s  but now with the same difficulty you need more than 60 days to get a bitcoin
thanks for the replys!

[baller1]

the block reward recently halved from a random maximum of 1 million to 500k, per block.

https://bitcointalk.org/index.php?topic=361813.0