Chuchin - have you considered the possibility that Ryanair have just written the policy to avoid taking on the cost of monitoring a minor during a flight (i.e. take responsibiliity for their safety ), but also ensure that they don't accidentally find themselves accidentally omitting to follow some rule and being panned in the tabloid press for what might have happened. I don't believe any LCC airline wants to consider what might constitute a suitable person - they just don't want the Daily Mail giving them bad publicity.
If you were trying to run a LCC, and someone possibly told you that to take unaccompanied minors you needed to have all cabin crew CRB checked to a sufficient level to look after children unsupervised, just so that you could carry the odd unaccompanied minor, what would your reaction be ? Mine would be 'sod the extra ticket revenue on a few kids, it's just not worth the additional costs and hassle'.