Allah, may He be exalted, in His wisdom has prescribed a variety of acts of worship for mankind in order to test them and show which among them are the best in deeds and most righteous. For people differ in their character and attitude. One of them may favour one type of worship because it suits him, but he may not favour another type because it does not suit him. So you will find that he does the former quickly and readily, but he does the latter slowly and reluctantly. The true believer is the one who submits to whatever will please his Lord, not what is in accordance with his whims and desires.
Among the varieties of worship are the various pillars of Islam. Some of them are purely physical, requiring effort and movement of the body, as in the case of prayer; some of them are physical but require refraining from things that are liked, as in the case of fasting; some of them are purely financial, as in the case of zakaah; and some of them are both physical and financial, as in the case of Hajj.
Hajj combines both physical and financial expenditure. Because it involves travel and more effort than other kinds of worship, Allah has only enjoined it once in a lifetime, and has stipulated that one must be able to do it. Being able to do it is a condition of an action being obligatory in this case and in others, but this condition of being able to do it is emphasized more in the case of Hajj more than in other cases.