First and foremost, the phrase ' lead us not' is a construction seen in poetry, not in prose. In the language of conventional prose, the Verse would  be constructed as :                                
                                    
 " Do not lead us into temptation, but deliver us from evil."               
                                    
 In case 'but' is substituted with : 'and', the construction would read as :            
                                           Do not lead us into temptation and deliver us from evil .      
                                   
  But that could end up being interpreted as ' Do not..' applying to both pieces of supplication. The usage of ' but' steers clear of such a misinterpretation that could  result from ambiguous construction.