Our law presumes innocence. The prosecution have to be able to prove guilt beyond reasonable doubt.
Previous convictions can't be brought in because that might sway the jury over and above the facts. Just because I drank the last beer in the house last week doesn't mean I did it this week, but you knowing that I did it last week will make you more inclined to think I did, and as the jury is basically opinion based then that's too much of a risk. Where they get brought into play is during sentencing, where the previous offences ramp up the severity of the punishment.
It's similar with the other charges in that the defence will argue that the more severe charges can't be proven. They can also do a deal where they say they'll plead guilty to a lesser charge which the prosecution may take to save risking not getting any conviction at all given the higher burden of proof required.
Ultimately, they couldn't prove intent to murder, so that takes out attempted murder. There are exceptions for carrying deadly weapons such as religious grounds, or work grounds eg cooks carrying knives, gardeners with machetes. In this case, I believe there was enough doubt to drop that charge. In the end, they got one to stick, which is the most important thing.