The end of support of SMF was the biggest reason for the switch to XenForo.
Honestly, XenForo is simpler at its most basic level of user interaction, with the reply box at the bottom of every thread and the ability to highlight text in someone's post then click once to automatically quote it in your reply. It also caches reply box contents so you can freely navigate a thread while writing a reply to it. Our SMF forum had a separate page for writing replies, with a differently-formatted preview of a limited number of previous posts on it. Quoting anything gave you the entire contents of the quoted post which you often had to pare down to what you wanted, and quoting less recent posts necessitated the use of a second browser tab.
XenForo is honestly better for the average user, at least those without nostalgia.
I agree that XenForo is better in terms of features, but depends on what you're after - SMF is usually used for smaller more close-knit communities as there's more charm in profile customisation and the lack of features meant that everyone was in the same place.
One of the biggest reasons for the decline in forum activity was A. Bannerlord being a terrible game and B. The inclusion of additional community features such as groups which spread out the already small Warband community.
I know first-hand that some of these features weren't wanted by the moderation team, but had no choice due to management wanting them, and still wouldn't change; self-moderating groups is ultimately silly as they can force post approval, which essentially means you can shut down any argument or not allow someone you dislike to post, which back in SMF, deleting posts in your own thread was warnable if no valid reason?
Lastly the forum colour scheme/layout is abominable, there's no real contrast of colours, everything is the same blue/grey as if I'm back playing Battlefield 3, there's numerous users from other forums such as FSE that have no idea what they're looking at and how to navigate, it's a mess. On top of that, there's loads of 'shortcut boards' scattered through the forums to redirect you to an area outside of your game? I expect to go into the Warband Multiplayer board and find tournaments, instead I'm greeted to a shortcut to a whole different board littered with tournaments from other modules too.
This isn't a dig at the moderation team, as I know for a fact a lot of the forum would change if they had the power to change it, but instead have to try to use loopholes to get around some piss-poor features added in by management.