Because SC2 was made using libraries of code which we are unsatisfied with. Better to make SC3 using good libraries.
When you make a software, you choose the language you want to write it in, and then, if you are like my brother, you choose to save additional time by using additional libraries. Libraries are pre-made functions made by other people using the code of your choice.
Often times, developers find themselves having to make a function over and over. A function is a group of commands which can be ran. Here is an example:
function kill(username){
//some code to set "Usernames" health to 0
}
So now instead of having to write the code to remove the user's health, a developer needs only write kill() with the username.
Often instead of making common functions over and over, programmers create their own "libraries" of pre-made functions, and then they share them with each other. This is a great way for a programmer to accelerate the time a project takes to develop. It has become common for developers to become experienced and skilled in several libraries-- therefor increasing their programming speed.
The problem comes when you are looking for new libraries to solve existing problems. You might try a new library only to find it's not very good, and wish to not use that library in the future.
SC2 used Faye.js in it's back-end which proved to be difficult to work with and likely is what led to the on going memory leak. Mithril.js is another library which was used on the front-end. It wasn't horribly, but it didn't contain enough structure and documentation, so we changed to vue, which is much easier for me.
I'd make the argument that we have learned over the years, which libraries are good to use, and which are not. I wish SC3 to be perfect, this is the first time we've worked full time on SC, and we learned a lot from SC1, SC2, and AC, so this will be our opportunity to apply what we've learned. It's important that this project be simple, neat, and work well because there is a lot of additional functionality I plan to add in the coming years.