Well, because it's true. You can't convert to Islam without denying Jesus. Sorry, that's the way it is.
How about addressing the issue of martyrdom now. Why are martyrs given high honor in Heaven, if one can convert under duress and it's not really a denial of Christ? Wouldn't that make the Christians in Rome and the Sudan a bunch of dummies?
And exactly how is it that you are so intimately familiar with what is involved in converting to Islam? Or is your knowledge merely conjecture? Are you basing your judgement on personal experience, or might you be bearing false witness against an innocent man?