Mansfield Park **
This is allegedly Austen's least popular work and Pride & Prejudice must surely be the most popular. Why? Comparison may be instructive.
P&P's romantic heroes are a dashing, rich, titled, educated and intelligent man and a pretty (but not the most beautiful), educated, intelligent woman who knows here own mind and insists on being appreciated for that mind. Mansfield Park's romantic heroes are a stick-in-the-mud boring but kind and principled second son likely to be comfortably off but not set to inherit the Estate and a timid, shy, submissive, boring girl who at least grows enough spine to not accept a loveless marriage to a morally defective but rich suitor.
The tone of P&P is one of wit, sardonic humour and sly social observation. There is little of this in Mansfield Park. It is replaced with a preachy moralising.
That's probably enough right there. I just don't think modern readers are nearly as receptive to the ideals presented by Fanny and Edmund as compared to those of Lizzy Bennet and Darcy and similarly, wit goes over better than sermons these days.
I struggled with much of the first 4/5ths, at times finding it hard to differentiate all the characters, especially the two Misses Bertram and to establish the connections between them all - especially so in the amateur theatrical week which proves crucial to all that comes later. Eventually I found myself intrigued as to how it was all going to resolve, making the final (sensational) fifth much more interesting.