The primary competitor would be Prodigy IMO, although that's not FOSS. Several open-source alternatives look interesting, such as Docanno, Brat, and Webanno.

In terms of what makes Markup better, I'd say it's mainly that:

- It's quick and easy to annotate with because it suggests complex annotations to you and will learn based on your feedback. Plus it allows for integrated querying of external ontologies, so if you highlight a span of text then it'll suggest matches from the ontology, ordered by similarity.

- It's flexible and configurable. For example, you'll soon be able to provide a small number of example sentences which will generate synthetic data to train a model for you (without any code), and those model weights can be saved for future usage.

- It includes a growing selection of in-built tools, such as a tool for statistically comparing two sets of annotations / annotators.

- It's possible to capture both simple and complex annotations. Most existing tools focus on classification or named-entity recognition, without giving users' the option of capturing attributes nor allowing them to annotate the same span of text repeatedly. Being able to do so enables the production of datasets for more complex use cases.