The Emperor Constantine who favored Christianity as early as 312, and convened the Council of Nicæa in 325, postponed baptism till 337. Did he provide any reasons for doing so?
I do not think anyone knows as there is huge speculation on the subject.
Basically some see it as his cowardice to confess his faith, others see it as a prudent measure to keep his influence of power until the risk of his own execution for becoming a Christian had passed. He himself does not seem to say. I do not even believe we can prove He was a Christian. Maybe he was? God only knows.
It does seem though the God used Him to end so much Christian persecution, and it does seem that in some ways, at least initially, Christ overthrew the Gentile world by turning his heart in favor of His people. Quite soon after his death, Christ's kingdom grew like a mustard seed.
The chief reason that people put off Baptism had to do with the Roman tendency to be almost magical in their thinking about religion. Even when they had converted in their hearts, people who had civic duties, which inevitably involved performing some pagan rites, were extremely reluctant to break with tradition. And Roman society in general didn't tolerate such omissions. So, a person could only convert when he could finally be sure he wouldn't have to perform such rites.
That didn't stop many of the famous martyrs, especially soldiers like St. Sebastian and St. Vincent. But as an EMperor Constantine simply could not do what was expected of him. So he hedged his bets, kept doing his imperial thing, and on his deathbed converted.