The main problem with this would be the server space and bandwidth required for this feature.
Let's say 100,000 players create maps with custom sounds. Let's say all of them upload 500Kb of sound files each, per map, which comes to 50000MB, or 50GB of sound files. These all have to be sorted into a database somewhere, which will continue to grow as more maps are made.
Now what if those users make 5 maps each? That's 250GB of space on the server hosting PB2.5 used - and that's 250GB of files that need accessing and searching every single time a map with a custom sound is played. This will add significant load to the 2.5 server, which will slow down the site and game loading. To compensate, a larger, more expensive server will have to be used - all just due to sound files.
Then on top of that, these files will have to be uploaded, then downloaded every single time the map is played, this will vastly increase the waiting times when loading into an MP match with custom sounds, or loading an SP map. The sheer amount of people downloading sound files from that server at once would consume an incredible amount of bandwidth. It is simply not cost effective to dedicate such a large amount of server resources purely to custom sound files for custom maps.
Another reason is abuse, if large files were permitted, players could upload offensive or abusive sound files such as recordings of insults or offensive language. Moderating custom sounds would be a highly tedious task, and would require a high volume of manpower.