Ease of use and portability.

The PEN cameras are unreliable - lots of problems are reported in connection with different models:

1. they do not have good ISO performance (lots of noise an UNEVEN distribution of light resulting in picture being lighter on one side - upper or lower, left or right - at ISO values higher than 1000).

2. Almost all models (E-PM1, E-PL2, E-PL3, E-M5...) cannot hold the "date, month, year, hour and minute" settings if the battery is removed from the body of the camera - as soon as the battery is removed (because of the need to be recharged) the "time" settings are lost and the user is faced wth the need to set it over and over again, each time the battery (BLS1 or BLS5) has been taken out and put back.

NEX-5 is twice as cheaper and 3 times better than E-M5!

The weather-sealing wins here...I've already shot in a rainstorm, and not having to worry about frying the camera or getting water in the lens was a huge advantage.

The 5N takes good pictures, but compare the two with the dpreview.com image comparator at high ISO (say, 6400 and above).  Notice how the chroma-noise tends to clump together on the 5N?  The E-M5 makes much better pictures under those conditions.