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.