Short answer: inventories are capped mostly because of technical limitations.
Long answer: the inventories of our players are the largest data chunk we have. It gets really large, really fast. Now, if this data was only sitting in our database, then sure, that'll be okay because we have plenty of storage space there. The problem is that this data is also sitting in the server memory - and we all know how awful Source is with memory. It's a crappy old 32bit application which means memory isn't plentiful at all. With all the other things our servers need to store in memory (and there's a lot of them), it's becoming a very tricky dance.
We haven't reached that point yet, obviously, but as we grow further and have more and more features, we will approach this dangerous point more and more. It's important to prepare for it from the start rather than panic about it later.
It's another one of these things where we're yet again screwed by the engine. Regardless, I'll monitor this and try to find a way to safely allow more space without risking memory problems.
Thanks for suggesting.