The difficulty has increased, and your daily mining is less profitable, simple as that
If you want to get technical, the difficulty has actually decreased the last two periods and is on pace for a third straight decrease:
Difficulty History
Date Difficulty Change Hash Rate
Dec 17 2014 39,457,671,307 -1.37% 282,449,013 GH/s
Dec 02 2014 40,007,470,271 -0.73% 286,384,627 GH/s
Nov 18 2014 40,300,030,328 1.76% 288,478,854 GH/s
Nov 05 2014 39,603,666,252 10.05% 283,494,086 GH/s
Oct 23 2014 35,985,640,265 2.81% 257,595,247 GH/s
Oct 09 2014 35,002,482,026 0.98% 250,557,526 GH/s
Sep 25 2014 34,661,425,924 16.20% 248,116,151 GH/s
Sep 13 2014 29,829,733,124 8.75% 213,529,547 GH/s
Aug 31 2014 27,428,630,902 15.03% 196,341,788 GH/s
Aug 19 2014 23,844,670,039 20.86% 170,686,797 GH/s
Aug 08 2014 19,729,645,941 5.30% 141,230,307 GH/s
Jul 25 2014 18,736,441,558 8.08% 134,120,673 GH/s
Jul 12 2014 17,336,316,979 3.08% 124,098,191 GH/s
Jun 29 2014 16,818,461,371 24.93% 120,391,236 GH/s
Jun 18 2014 13,462,580,115 14.51% 96,368,902 GH/s
Jun 05 2014 11,756,551,917 12.44% 84,156,677 GH/s
May 24 2014 10,455,720,138 18.10% 74,844,960 GH/s
May 12 2014 8,853,416,309 10.66% 63,375,223 GH/s
Apr 29 2014 8,000,872,136 14.64% 57,272,474 GH/s
Apr 17 2014 6,978,842,650 14.04% 49,956,502 GH/s
Apr 05 2014 6,119,726,089 22.23% 43,806,706 GH/s
Mar 24 2014 5,006,860,589 17.80% 35,840,504 GH/s
Mar 13 2014 4,250,217,920 11.39% 30,424,245 GH/s
Feb 28 2014 3,815,723,799 21.92% 27,314,015 GH/s
Feb 17 2014 3,129,573,175 19.39% 22,402,357 GH/s
Feb 05 2014 2,621,404,453 19.49% 18,764,744 GH/s
Jan 24 2014 2,193,847,870 22.59% 15,704,175 GH/s
Jan 13 2014 1,789,546,951 26.16% 12,810,076 GH/s
Jan 02 2014 1,418,481,395 20.12% 10,153,885 GH/s
Dec 21 2013 1,180,923,195 30.01% 8,453,378 GH/s
Dec 10 2013 908,350,862 28.41% 6,502,229 GH/s
Nov 29 2013 707,408,283 16.07% 5,063,826 GH/s
Nov 17 2013 609,482,680 19.29% 4,362,847 GH/s
Nov 05 2013 510,929,738 30.70% 3,657,378 GH/s
Oct 26 2013 390,928,788 46.02% 2,798,377 GH/s
Oct 16 2013 267,731,249 41.45% 1,916,495 GH/s
Oct 06 2013 189,281,249 27.19% 1,354,928 GH/s
Sep 25 2013 148,819,200 32.13% 1,065,289 GH/s
Sep 14 2013 112,628,549 29.56% 806,227 GH/s
Sep 04 2013 86,933,018 32.22% 622,291 GH/s
Aug 24 2013 65,750,060 29.40% 470,657 GH/s
Aug 13 2013 50,810,339 35.88% 363,715 GH/s
Aug 03 2013 37,392,766 19.63% 267,668 GH/s
Jul 22 2013 31,256,961 19.47% 223,746 GH/s
Jul 11 2013 26,162,876 22.63% 187,281 GH/s
Jun 29 2013 21,335,329 10.32% 152,724 GH/s
Jun 16 2013 19,339,258 23.92% 138,436 GH/s
Jun 05 2013 15,605,633 28.41% 111,709 GH/s
May 25 2013 12,153,412 8.64% 86,998 GH/s
Though yes, the point you were trying to make is network hash rate growth is exponentially increasing, even with the 3 most recent difficulty decreases, though it seems we may reach a point of equilibrium in the not so distant future.
For the noobs, there's too many dudes mining the same gold nugget, that you only end up with gold dust on your pickaxe. Biggest pickaxe gets the biggest chunk.
