What version of Premiere Elements are you using and on what computer operating system is it running?
The Premiere Elements built in YouTube feature posts limits as to file size and duration. If you have an upload greater than the posted limit, then you can export your Premiere Elements Timeline content to a file saved to the computer hard drive from where it can be uploaded to YouTube at the YouTube web site. The YouTube web site also has limit, unless you apply there for an extended time account. These extended time accounts do not work in the Premiere Elements built in YouTube feature. You probably came into this thread knowing all that. The why that you seek needs to come from Adobe. And, we are not Adobe here. Rather user to user.
You can file an Adobe Feature Request Bug Report Form on the matter.
This type of situation is not specific to just one version of Premiere Elements.