I don't subscribe to the belief that The Lion Guard was "cancelled" or "cut short." Just based on tons of evidence, the show's production had a natural lifetime (especially given that it lasted for 7 years - from February of 2012 to July of 2019). The show had a very clear beginning, middle, and end, and that was that.
That being said, I do believe this show ultimately suffered from how the network treated it. Because the episode distribution within the latter half of the show was abysmal. Crash-out approaching in 3...2...1...
Season 2 had a run-time of two years despite only having 3 more episodes than Season 1. The reason being is because of how several episodes in the second half of Season 2 were stretched out by 3-4 months in between small batches of 3-4 episodes. And keep in mind, the production team doesn't call the shots on episode premieres, the network does - one of the animators on the show expressed disappointment in how slowly the network was pouring out episodes and stated that it "killed momentum"
Low and behold, they were right. Season 2 premiered with 1.02 million viewers, and ended with 0.44 million. You'd think the season with the most amount of episodes and several guest stars would've boosted views, but the bad release schedule gave it the opposite effect.
This is a pet peeve I have in general with a lot of networks that distribute/air children show's like this: Your audience is predominantly small children, a demographic that is notorious for their lack of patience (they're smarter than we give them credit for, yes, and can learn such a trait, but many things can be true at once).
And unlike teenagers and adults, they aren't as active on social media where they can get updates on news outlets or from creators online. Heck even teens and adults will get fed up with constantly waiting months on end to see small batches of episodes that were long-since completed.
And honestly, even with that major fuck-up, the network could've redeemed themselves from it with Season 3. Remember, the show wrapped up animation in March of 2019 and was archived in July of 2019. Either way, that was enough time for the network to receive episodes. They had a chance to apply a consistent release schedule; one that may not have saved the viewership after Season 2, but would've at least managed to stabilize for the home stretch and give the show enough footing to have more time on the air after its lifetime.
So what did they do? They kept fucking with it
They released the final 20 episodes online in the span of a month before any of them aired on television!!!
Season 3's release had the opposite problem as Season 2's: They blew their load with releasing the season, and directed all of their promotion to the DisneyNow streamings rather than the television airings. I don't even think there was a promo for the finale. Now compare that to the insane promotional campaign for the pilot movie, and you can see just how lazy everything got.
As one can expect, people (especially kids, the target demographic of the show) watched the last episodes online and then just dipped after that, not sticking around long enough to see the episodes air on television. And this was another huge kick to the nuts for the ratings, as the show ended with 0.46 million viewers. A net loss of 4.8 million viewers over the span of 4 years.
The real kicker? Here in the U.S. the show was allegedly pulled from Disney Junior on December 31st, 2019, 8 weeks after it ended. The show's viewership was so bad that the network couldn't even keep it on the air during fucking New Year's Eve - when everyone was at home doing nothing but watching TV!
Also, I largely blame Season 2's snail-paced airing for the reason why Season 3 doesn't have any merchandise. Disney wanted to focus products for the 2019 remake, so there were no new products for Season 3. Had Season 2 been given a better release schedule, Season 3 might have wound up airing earlier (like from late 2018 to early 2019 or just early 2019) - this would've meant that the company would've been given a chance to release products for the show and the remake: Giving us merchandise of the aged up designs of the Lion Guard.
So yeah while I definitely think that the show wasn't cancelled and had it better than other shows in terms of being allowed to get a proper conclusion, it got the short end of the stick from the network's really trashy way of releasing episodes.