What is so appealing about Christianity?

I think you've missed some key points in your imagined scenario.

It wasn't "a simple message of love" spread by a few ordinary fisherman. Jesus wasn't some hippie revolutionary who got a bunch of followers to parade around telling everybody they should just love each other.

What made Christianity appealing was that it was true. This would have been much more apparent to the early followers. As time went on this has gotten increasingly hard to see, but lets look at the scenario again.

1. Ordinary fisherman spend a couple years hanging out with this dude Jesus.
2. Said dude wows some crowds but ends up on the wrong end of the political stick.
3. Said dude gets executed (in dubious circumstances) for blasphemy/treason.
4. Ordinary fishermen go back to their nets.

Obviously there are more layers to this. What Jesus was actually teaching during this time, the addition of miracles, and quite a number of other factors are significant and should be given more treatment than I will here. The reason I filtered the scenario down to just these factors is to highlight one significant factor I think is underestimated.

The return to fishing is not an unexpected move. The fisherman were disillusioned, nobody really wanted to get strung up for association with a recently executed rebel and they didn't really have anywhere else to go. Bread wasn't going to put itself on the table and they no longer had (in their own estimation) a message that was going to change the world. Their own leader had failed — even let them down — and who were they to take up his mantle?

Then the resurrection happened.

Acts 2:22-24 (ESV)
²²  “Men of Israel, hear these words: Jesus of Nazareth, a man attested to you by God with mighty works and wonders and signs that God did through him in your midst, as you yourselves know— ²³  this Jesus, delivered up according to the definite plan and foreknowledge of God, you crucified and killed by the hands of lawless men. ²⁴  God raised him up, loosing the pangs of death, because it was not possible for him to be held by it.

These fishermen weren't going to be taken in for a hoax. The real flesh and blood dude that they'd spent three years traipsing around the countryside with had just shown himself master over death itself. They could hear him, touch him and he ate their fish.

The history of every nation revolves around Jesus of Nazareth. This is really an astonishing claim: to say everything depends on a guy who lived two thousand years ago, ate some fish, and got himself killed. And then ate some more fish. — Telford Work

What happened next is truly epic. After a couple weeks Jesus left the scene, heading back to his father. The disciples were now left with something completely different than what they had after three years of just hearing hearing the guy teach.

This time instead of heading back to their nets, they committed themselves to telling the world about this dude at the cost of hardship, torture and eventual death. To the last man.

These men would not have thrown their lives away for anything they were not absolutely convinced was true. In some circumstances it is possible to be absolutely convinced that a lie is true, but these men were were in a position to know if the guy was a fraud. And quite honestly they had their doubts for a while, but in the end nobody, least of all themselves had a motivation to perpetrate a lie.

• The leading Jewish religious leaders would liked nothing more than to have pointed to his grave.
• The Roman solders guarding his tomb would want to keep their heads. Attached to their bodies.
• The disciples who the Jewish leaders tried to blame with a conspiracy would have the least reason of anybody to actually pull one off. And if it had been them, which of them would have gone to his death defending something he knew to be a lie that would have netted a fortune to revel.

Of course the power of God through his Spirit made this possible. But at heart it wasn't a "message of love" that was irresistible. It was truth.

Acts 2:36 (ESV)
³⁶  Let all the house of Israel therefore know for certain that God has made him both Lord and Christ, this Jesus whom you crucified.”