There’s a brilliantly tongue in cheek article on Vagabondish laying out the reasons why:

Step One: Get Crazy Cheap Airplanes!
Step Two: Staff Almost No One
Step Three: Don’t Waste Money Training Aforementioned Staff
Step Four: Punish Passengers for Bringing Luggage
Step Five: Distract the Passengers In-flight
Step Six: Arrive on Time So Nobody Can Say Otherwise (and Mum’s the Word!)

Vagabondish – A Tribute to RyanAir: Decoding the Perfect Business Plan

If you want more, get the book Ruinair by Paul Kiduff.

My first and last flight on this airline was a comic experience.

RyanAir does not provide those small luggage tags … but still requires your name and address on luggage. I was forced to write my name on my pack in ink.

Some of the airports they use have no other commercial airlines. And almost no services.

Worst of all, you must print your own bording pass before getting to a RyanAir flight. If you forget, that will be a 40 Pound ($64.37) surcharge!

It’s the only airline that actually weighs your one allowed carryon. Maximum weight 10kg (22 pounds).

Their biggest competitor in Europe is EasyJet, not perfect, but far better than RuinAir.

The best discount airline I know is WestJet in Canada.