Part of it is volume. HNSearch suggests medium got 425 submissions in the last 4 weeks. That's a higher submission volume than TechCrunch (327), ArsTechnica (193) or the BBC (195) or even The Guardian who broke most Snowden/NSA news (181).
Belying this volume is an almost complete lack of quality. The most popular 10 articles in the last 4 weeks from medium and the above sites:
medium:
150 136 107 47 32 28 17 16 14 13
TC:
462 183 175 146 141 120 113 99 80 74
Ars:
524 316 271 212 183 170 149 122 121 109
BBC:
915 167 166 123 110 93 81 75 59 58
Guardian:
388 328 293 185 184 183 182 156 162 149
The best of medium is just not very good anymore and it's a sharp and sudden fall into very forgettable content which makes you wonder why it's even submitted let alone so much.