The support of a political party for any government initiative depends on several factors. The merit of policy is hardly any issue. Let us analyse various factors that influence the decision of a political party.
1: Government-Opposition Dichotomy
If your party is part of the government, you are likely to support almost all decisions of the government except when it hurts your own constituency. The same decision has to be opposed when you are in opposition.
Mamata Ji is in opposition, hence she has to oppose the decision.
2: Personal Equation
She does not share good relationship with BJP. She has personal grudges against Shri Narendra Modi due to the statements made by each other in the past. She also feels that Modi Government is using agencies like CBI to target her.
Hence, she would oppose all moves which bears the stamp of Shri Narendra Modi.
3: Political Gain
If a party feel that opposing a move would benefit it politically, the leaders oppose the move. She is perhaps convinced that demonetisation move is causing lots of inconvenience to the common people. Hence she hope to get their support by opposing the move.
4: National Interest
Sometime political parties support the move forgetting all personal benefits/loss if the move is in national interest. For example, Shri Nitish Kumar and Naveen Patnaik have supported the move despite being political rival of BJP. However, this is very rare in Indian politics. She perhaps doesn’t see national interest in the move of the government.