I would say that it has to do with the fact that ‘sin’ and ‘grace’ are actual realities, with actual effects. The Eucharist isn’t magic – it works in us and through our ability to receive the graces present in it. Therefore, the more you are disposed toward grace, the more grace you are able to receive from the Eucharist. Of course, at the far end of the scale, if you are not disposed toward grace – that is, if you are in a state of mortal sin – then you cannot receive graces from reception of the Eucharist.
‘Sin’, too, is a reality. To deal with sin, the Church provides sacramental reconciliation. One goes to confession not just to say “I did it”, but to actually restore his relationship with God. Without that restoration, it’s not possible to benefit from the Eucharist.
When I was a kid, there were tons of gasoline additives on the market – some claimed to clean a car’s inner workings, others claimed to improve gas mileage, etc, etc. Now, you could put all the ‘additives’ into your tank that you want – but if the tank was empty of gas, then they did no good. It’s a simplistic analogy, but I think it holds on a basic level: without some level of grace present in a person, he cannot benefit from reception of the Eucharist. Only the restoration of some level of ‘gas in the tank’ (i.e., some grace restored through confession) can allow the healing and strengthening benefits of the Eucharist to do their thing…