We have given tribute to Gandhiji for the path he showed us, from start to end.....till independence. And being professional, having one father of nation, we feel its better to have one type Notes. Afterall giving tribute to great other leaders has got many other ways apart from their photo on the currency. We have many organisation, NGOs in their name, We aren't Americans though our era is definitely following their track....having diverse currency notes will add to it.
Morover, having multiple notes, will diversify the currency and thus chances to illegal notes will also increase, along with inc. level of ignorance. People will say, Note asli hi hoga, humari govt. ne itne notes chaap rakhe hai....nobody will have correct knowledge as always the case is. Actually when things are established and are widely accepte without needing a change, they should't be touched unless improvement is guranteed.
When we got Independent, their was a strong fire of new feeling, sooner the bad news of Gandhiji's death provoked the people. They showed their tribute by adding such a currency thing into the constitution, since then, we are following what doesn't need a change.