They have lots of ready to go connectors which makes it easy to integrate lots of apps instead of figuring out all the SAML settings... but any SAML provider will work as long as you can map the right fields. But people like easy things and Okta makes it easy. Many apps who don't have a connector will have docs on how to set things up with Okta, for others you'll have to take the Okta instructions and figure out what that means for an alternate provider. There are other things like counter-signatures which may be easier to do with Okta but also possible with others, if you have the right docs.
People use them because they are easy to use, on the other hand their security issues over the past few years could make clients consider alternatives.
