There's probably a team of two developers on £24k a year been given the job of launching the on-line licence renewal service. It works fine for most boaters and the team will be working hard to analyse and fix the cases where it didn't.
The other way would have been for CRT to throw £1m at the project to get design it properly and get it thoroughly tested and debugged before launching. People would then complain this money should have been used to fix leaky lock gates etc.
If renewing on-line doesn't work for you, ring them up and renew on the phone. Or send back the forms. Both these methods work fine for most of us, most of the time too. Just like any other system there will always be the fringe cases that don't work.
I bet if someone could design and set up a poll where the respondents are not mostly the self-selecting dissatisfied, 99% of attempts to renew on line will be found to be successful.