I'm all for changing up a stage production so you're not simply putting on stage what's been in the film (questioning the point of the production really) but ,yes, Mrs Banks was boring in the production...... I think Glynis Johns set the bar very high in the film and she's so memorable that you're bound to make comparisons.
I think they're trying to place more emphasis for the audience (especially for kids etc) on Mary. The suffragette/big mrs banks character element may undercut the Maryness of the show.... if that makes any sense at all. Similarly they need to emphasise her not being able to fully stand up to Mr Banks