The goal of the trade system is to make short, secure transactions between users, not to move your entire inventory around. If you don't want your items to be on this account, maybe consider using your other account for participating in events?
The item limit is a deterrent; if some malicious person had gotten ahold of your account and wanted to send all of your items to their own account, they are less likely to do so if they have to use multiple batches (and reenter passwords as you are required to). It is also easier to track where items are going, when, and how many, and remedy any errors, if the items are sent in smaller batches. We want to make sure that all the items you are trading are going to the correct users, and everything runs smoothly.