1. BJP income rose after demonetisation. Parties don't pay taxes but file return for exemptions, expenses, etc. You don't even have to reveal the sources. https://www.dailyo.in/politics/demonetisation-noteban-party-...

2. If a party already knew that demonetisation is coming, it could have acted accordingly. https://www.financialexpress.com/india-news/demonetisation-b...

3. Ruling party had an advantage on getting money, organisational structure, advertisement, etc, etc. Many vendors don't accept checks but real currency.