It just can't. There is no real reason to why, gamefreak just decided that it doesn't learn it. If you need a fighting type move faster than Focus Punch then you could train your Poliwrath until it learns Dynamic Punch (at level 43 if I remember correctly), but its not exactly very reliable due to being only 50% accurate. Alternatively, you could try to get Brick Break from the Battle Frontier shop or Focus Blast from Goldenrod City store. Focus Blast isnt the most reliable move either though as it is just 70% accurate. It can also learn Vacuum Wave from a move tutor, but it is quite weak since it is a priority move. Last but not least, move relearner will actually be able to make your Poliwrath learn Submission.
In short your options are:
Dynamic Punch (100 BP, 50% acc, always confuses)
Brick Break (75 BP, 100% acc, removes Reflect and Light Screen)
Focus Blast (120 BP, 70% acc, can lower sp. def)
Vacuum Wave (40 BP, 100% acc, priority)
Submission (80 BP, 80&% acc, does recoil damage)
Personally I would recommend getting Brick Break