请同学们阅读下面短文，然后完成后面的阅读任务。
TheMississippiRiver
ForHuckandJimtheMississippiRiveristheultimate(完全的)symboloffreedom．Aloneontheirrafttheydonothavetoanswertoanyone．Therivercarriesthemtowardfreedom：forJimtowardthefreestates;forHuckawayfromhisabusive(虐待的)fatherandtherestrictive(限制性的)“civilizing”ofSt．Petersburg．MuchliketheriveritselfHuckandJimareinflux(不断变化着)willingtochangetheirattitudesabouteachotherwithlittleprompting(鼓励)．Despitetheirfreedomhowevertheysoonfindthattheyarenotcompletelyfreefromtheevils(邪恶)andinfluencesofthetownsontheriver’sbanks．Evenearlyontherealworldintrudes(侵入)ontheparadiseoftheraft：theriverfloodsbringingHuckandJimintocontactwithcriminalswrecksandstolengoods．ThenathickfogcausesthemtomissthemouthoftheOhioRiverwhichistobetheirroutetofreedom．
Asthenovelprogressesthentheriverbecomessomethingotherthantheinherently(天性上)benevolent(慈善的)placeHuckoriginallythoughtitwas．AsHuckandJimmovefurthersouththedukeandthedauphin(皇太子)invadetheraftandHuckandJimmustspendmoretimeashore．Thoughtherivercontinuestoofferarefuge(避难所)fromtroubleitbringsthemtodriftsouthwardtowardtheDeepSouthandentrenched(确立了的)slavery．AsHuckandJim’sjourneyprogressestheriverwhichonceseemedaparadiseandasourceoffreedombecomesmerelyashort-termmeansofescapethatpushesHuckandJimeverfurthertowarddangeranddestruction．
Question：
WhydoHuckandJimdriftontheRiver?