Software fossilizes once it reaches a certain popularity. And while the companies will often add new features they will usually not touch the core functionality, as that is extremely risky. From the perspective of a company with a working software product it's always better to add incremental improvements than to refactor large core features, as most large changes will increase customer churn.

And that's how new incumbents get a chance to disrupt existing solutions: Their risk-reward calculation is vastly different so they can be much bolder when experimenting with core functionality, which sometimes leads to a product experience that is noticeably different (not always better) than existing products. Also, incumbents can use cutting edge technologies to build their products whereas existing solutions are burdened with their "legacy" tech. Newer is not always better but in certain areas (e.g. frontend development) technologies have progressed quite a bit and that gives incumbents an advantage.

Also, the market for software products is still growing very fast as more and more companies start using cloud-based software to organize work, so there's a large influx of customers that have never used any task management software before.