Why did the Prophet link the obligation of Hajj to "if I had said yes, it would have become obligatory and you would not have been able to bear it," even though the principle in his commands is that they are according to one's capacity, as in the Hadith about the siwak?

The aforementioned Hadith is narrated by Abu Hurairah (may Allah be pleased with him). The meaning of the Prophet's (peace and blessings of Allah be upon him) saying, "If I had said yes, it would have become obligatory, and you would not have been able to do it," does not mean negating the ability of the entire Ummah. Rather, it means that many would not be able to bear the obligation of Hajj every year. It might become difficult for those who cannot afford to miss it, and difficult for those who can afford it to go every year, thus incurring sin if they miss it. The Prophet (peace and blessings of Allah be upon him) was sent with the tolerant Sharia, as Allah Almighty said: {There has certainly come to you a Messenger from among yourselves. Grievous to him is what you suffer; [he is] concerned over you and to the believers is kind and merciful} [At-Tawbah: 128]. The obligation of Hajj every year involves hardship, and it has been called Jihad due to the parting from homelands and the arduousness of the journey. The meaning of "and you would not have been able to do it" refers to going for Hajj every year due to its difficulty, not that everyone would be unable, but rather that this would apply to many among his Ummah.