I believe it was just a different feature set/market they were looking at for that unit. With something this small sometimes they have to pick & choose which features to include for cost and/or space reasons. If you're looking for an i3 NUC with on board LAN the DC3217IYE is the previous generation NUC model, while the D34010WYK is the Haswell version you'd want.
If you need an i3 with Thunderbolt AND ethernet the DC3217BY there are a number of Thunderbolt to Ethernet adapters available. Here's an example: http://www.amazon.com/dp/B008ALA6DW/?tag=googhydr-20&hvadid=36162757840&hvpos=1t1&hvexid=&hvnetw=g&hvrand=7077107702100505229&hvpone=&hvptwo=&hvqmt=b&hvdev=c&ref=pd_sl_4nlpgigfdx_b
I hope this helps.