When you upload the data via the UI, Splunk detects the source file format and assigns a sourcetype that makes sense. In this case, probably _json
If you want to achieve the same result via the command line, you need to configure and specify a sourcetype with the proper settings for your json data. When you are using the command line, you have to replace some of the smarts of the UI with manual actions.
I'd recommend reading (at least) this chapter of the Getting Data In manual to understand how Splunk processes data.
You can also go through the UI once, then save the sourcetype settings under a new name you chose, and then use that sourcetype on subsequent CLI uploads.
