Why was the supplication that includes invoking God through His attribute of love forbidden, even though it is considered invoking an attribute of God and not invoking the prestige of believers?

The gist of the Fatwa is:

We did not deny that Allah’s love for His Prophet is one of His attributes, but we advised the questioner to supplicate with what is indisputably legislated, which is supplicating with the Beautiful Names of Allah or interceding through good deeds. We also draw attention to the fact that interceding with something that is not of one's own doing is a matter of dispute. The relevant point in Shaykh al-Islam's statement is his saying: "Whoever asks by something external to him that is not a cause for his benefit," to the end of his statement. Caution in matters of worship, including supplication, dictates refraining from what has not been proven and sufficing with what has been established.