In order to understand why the HOMO of the dienophile is less easily donated, let us again understand the original Diels-Alder properly.
In the original Diels-Alder reaction, there is a close similarity in the energies of the HOMO of the diene and the LUMO of the dienophile. As a result, they form the strongest orbital interactions. The presence of electron releasing groups on the diene and electron withdrawing groups on the dienophile essentially reduce this energy difference even further.
Moving on to the inverse electron-demand Diels-Alder: here, the LUMO of the diene and the HOMO of the dienophile are closest in terms of their energies. Because of this, they form the strongest orbital interactions now, and hence this reaction is able to take place.
As you might have already guessed, the chances of there being a close similarity in the energies of the LUMO of the diene and the HOMO of the dienophile is much less compared to the normal Diels-Alder case. Because of this, we have a greater number of reagents that can undergo the normal Diels-Alder than the inverse one. It's just a question of its rarity than its feasibility, because now you're looking for an electron deficient diene and an electron rich dienophile. In general, due to the resonance stabilisation in dienes, there's a good deal more electron rich dienes than deficient ones, and vice versa for the dienophiles...
I hope that this answers your question.