Because he thinks he's found some problems with the play. Here's what he says:
There are things in this comedy of Pyramus and
Thisbe that will never please. First, Pyramus must draw a sword to kill himself; which the ladies cannot abide.
The ladies will be frightened of Pyramus' suicide. So Bottom's solution?
Write me a prologue; and let the prologue seem to say we will do no harm with our swords, and that Pyramus is not kill'd indeed; and for the more better assurance, tell them that I Pyramus am not Pyramus, but Bottom the weaver.
A prologue which will explain that Pyramus is really Bottom, and that the swords do no harm, and Pyramus isn't really dead.
Masters, you ought to consider with yourselves to
bring in—God shield us!—a lion among ladies is a most dreadful thing...
The ladies will also fear the lion. So Bottom's solution is another prologue, spoken before the play, to explain that - you guessed it - the lion isn't really a lion, spoken by the lion:
Nay, you must name his name, and half his face
must be seen through the lion's neck; and he himself
must speak through, saying... If you think I come hither as a lion, it were pity of my life. No, I am no such thing; I am a man as other men are.’ And there, indeed, let him name his name, and tell them plainly he is Snug the joiner.
Bottom claims that this isn't a prologue when Snout suggests it - but you rather suspect he just likes his ideas only.