1. Because He meant it.
2. Because all who believe receive eternal life, which is a gift (Rom 6:23) which is irrevocable (Rom 11:29).
Any other reasons?
Irrevocable? In context your verse says this about being irrevocable.
Romans 11
25Lest you be wise in your own sight, I want you to understand this mystery, brothers:a partial hardening has come upon Israel, until the fullness of the Gentiles has come in. 26 And in this way all Israel will be saved, as it is written,
"The Deliverer will come from Zion, he will banish ungodliness from Jacob"; 27 "and this will be my covenant with them when I take away their sins."
28 As regards the gospel, they are enemies of God for your sake. But as regards election, they are beloved for the sake of their forefathers. 29 For the gifts and the calling of God are irrevocable.
Who's gifts are irrevocable? Yisrael's. What will Messiah do? Who is the covenant made with 'that takes away Yisrael's sins'?
I see your quite adjusted to using verse out of context.
Romans 6
22 But now that you have been set free from sin and have become slaves of God, the fruit you get leads to sanctification and its end, eternal life. 23For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord.
Context
John 3:15 ESV
that whoever believes in him may have eternal life.
- John 3:16 ESV
[For God So Loved the World] "For God so loved the world,that he gave his only Son, that whoever believes in him should not perish but have eternal life.
- John 3:36 ESV
Whoever believes in the Son has eternal life; whoever does not obey the Son shall not see life, but the wrath of God remains on him.
- John 5:24 ESV
Truly, truly, I say to you, whoever hears my word and believes him who sent me has eternal life. He does not come into judgment, but has passed from death to life.
- John 6:47 ESV
Truly, truly, I say to you, whoever believes has eternal life.
You say faith comes by God alone. So God creates puppets who either have the fuel(means) to succeed or not? Let's see what the bible say about where faith comes from.
Romans 10
13 For "everyone who calls on the name of the Lord will be saved." 14 How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? 15 And how are they to preach unless they are sent? As it is written, "How beautiful are the feet of those who preach the good news!" 16 But they have not all obeyed the gospel. For Isaiah says, "Lord, who has believed what he has heard from us?" 17 So faith comes from hearing, and hearing through the word of Christ.
Eph 1
13 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
Faith comes by hearing the words of God. Yet, we can even believe the words of those who witnessed the words of God too.
John 17
20"I do not ask for these only, but also for those who will believe in me through their word,21that they may all be one, just as you, Father, are in me, and I in you, that they also may be in us, so that the world may believe that you have sent me.
So faith comes by the human function of hearing the word spoken from heaven, be it by Messiah himself, an angel, or those who witnessed these words spoken. Faith is not miraclously bestowed and maintained by God in our lives. We are responsible to 'listen' and 'repent'. Why else did Jesus continually say 'you are forgiven your sins, no go sin no more (repent)'?