The reason is a financial one.
This site still does not pay for itself. It's funded entirely by me, helped by donations and sponsors. Costs (From the software and hosting provider) and are based on the number of currently viewing members over a 24 hour period. i.e. currently, on average, at any one time we have between 300-400 users on the site, 24 hours a day. This is in addition to AWS Amazon media hosting and the email provider too. its roughly £350-400 per month
As subforms are added, and they go behind a login I am hoping that the registered users will become more likely to be ongoing members of the community, and possibly help support the community rather than viewers from afar. Also, it's easier to show sponsors that we have xxx logged in and active members.
If people are not interested in being members of our community then they will not visit as and the hosting costs may come down. I would prefer an amazing community with 100 active members that really support each other and love what we are all building here, over 60,000 casual readers (this is our unique monthly visitor figures)
I will also be putting Google Adsense display ads all over the site that will be visible to only non-logging in users. I'm not a fan of this as it doesn't look great, but with more logged in users it will
Its still free for members, its just going to need a log-in to access parts of the site.