Another way of looking at this could be: Why wasn't stuff written by the other apostles included? The apostles who spread in basically every direction from Jerusalem, and spread to sub-Saharan Africa, and even into India, surely they wrote stuff worth including?

There's no evidence that the other apostles wrote anything else. If there were texts circulating early that had been accepted by the early church as authentic (whether they were or not), they probably would have been included, but there's simply no such texts. And there's no reason to necessarily suggest that they did write.

Why Paul, and why so much?

Consider: Paul appears to have been quite well educated, from the Jewish perspective, and probably of reasonably good background in a hellenistic city and exposed to some hellenistic learning. Paul also works as an itinerant, visiting cities where he establishes local christian communities where he spends some time, up to 3 years in one case, 'kick-starting' those groups. He then engages in letter-writing to these communities while absent, very often to address local issues. This is why Paul wrote so many letters. The other apostles (a) do not appear to have followed such a strategy, (b) if they did 'spread' so far, did so relatively late in life, and (c) do not appear to be prolific Greek-language letter-writers.

Paul's letters, meanwhile, were valued enough that they were circulated and copied among christian communities, gaining currency as valued texts, and eventually inspired and authoritative ones.