Can think of many reasons! I've had a Eb for 18 months now & its epic. For me, at the time I ordered the cost of the V8 was too much, add insurance & running costs based on the fact I was planning on about 10k miles pa in it & the Ecoboost was the logical choice.
It returns about 33mpg on average, so it's far cheaper to run. It's ***** rapid, trust me. Sounds pretty good inside IMO. Lighter in the front end because of the smaller engine so handles well (better than the V8 some say), and well, it looks just as stunning as the GT.
Having said that, the biggest drawback? It's not the V8. The beautiful burble is not there, which having spent a lot of time around them in the last year or so, it's something I really want. Power wise, the EB is fast but lacks that little extra oomph that the V8 has. Can be achieved with a tune & various other bits but will never have the same appeal as the V8.
As it happens, I caved earlier in the year, placed an order for a GT which I should have in my hands in the next 6 weeks or so. So my Ecoboost is up for sale, will be very sad to see it go but I'm hopeful that someone else will be able to enjoy it as much as I have...