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 one.
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.