Developing your own internal tool can be done if you can spare resources, but most of the time it will be done half-baked. Statuspage.io provides you with minimal overhead - unless you can spare dev resources, do your own A/B testing, ensuring it's up, managing another internal tool for your status pages takes up a lot of work. Another git repository to manage, another project held in everyone's cognition.
During downtimes, usually everyone is poured into solving a problem. Not updating and doing status pages.
You can say that Statuspage.io can be replaced with a wordpress website at it's most basic functionality, but even that wordpress website needs someone to take care of it, to make sure it is up and running during your lowest points. Extra stress, extra thing to worry about.
For anyone who runs a huge web business (see their list of clients), $79USD for a reliable service that acts as a crucial part of your system that faces customers during the lowest points of your service is pretty cheap. Compare it to the hundreds, if not, thousands of dollars you may lose in damage control during a downtime, just because your customers had no updates on what the hell is happening.
PS: I do not work for statuspage.io
I often see eg Pingdom take flak for being expensive. However, their basic account is a mere $10, and $40 for their pro (a lot less if you sign up for a year). It's a trivial cost for them to offer the exact same status page product, just stapled on to their existing offerings. With their massive customer base, it's also very easy for them to instantly dominate the 'status page' market, and make it all but impossible for StatusPage.io to thrive long-term (assuming a good product).
Given the cost of offering these types of services is navigating toward zero per unit, in my opinion there's no chance a company can hold a $80 per month price tag.
http://status.kissmetrics.com/
My goodness that's pricy. They do a good job of making their case by sharing the status pages of their customers. However, if I was one of those customers, I'd expect to be given a free account. After all, I'm baring my important stats so that you can sell more subscriptions.
For internal reporting, the high level overview that a typical status page gives seems pretty useless.
https://www.pingdom.com/pricing/
Disclose: I Do Not work for pingdom