To do a quick summary as far as I understand it-

Both promise equality before judgement and in the waiting list for heaven. Your wealth in this life is irrelevant. Actions that get you to heaven are what we would all consider “good” across all cultures. Further there are no “castes” or untouchables vs say Hinduism or Zoroastrianism.

They are organised, common rituals and practices, solid source material and on the basic level not hard to understand across cultures. This was a major issue for pagan religions. These rituals build cohesion and a sense of a wider community.

They are not limited ethnically. They are for all mankind. Norse or Greek religion (even Zoroastrianism) was really concerned with the local people.

They were also in the right place at the right time. Islam bursts on to the scene to decisively defeat the weakened Byzantine and Persian empires (though it is being foolish to say it still wasn’t a near miraculous feat). Overtime the incentives to being a Muslim outweighs staying as your current religion etc.

Christianity was oppressed but for reasons above it continued to spread and served as an untapped political resource in the cities where Christians were concentrated. Constantine taps into this, though he probably had sympathies to their beliefs to not be over cynical.