But x64 also gives access to many more registers, which is a tradeoff the more then counteracts the increased memory footprint of pointers, especially in threaded workloads.There's also people who think that 32-bit is faster if you have 4GB or less of RAM, which is still very common for most PCs built before this year. It actually _is_ faster for some programs, hence the whole x86_32 architecture thing that Google was pushing (smaller pointers means data structures are smaller means more data fits in cache and can be retrieved with each memory access means better data throughput). These folks might intentionally be avoiding 32-bit still. Older hardware is popular with Linux users, and in general cutting-edge hardware is geared more towards gamers who are of course not generally Linux users on account of almost every major PC title still being Windows only.
Keep in mind, Linux has been starting to drop older H/W at a very alarming rate (ATI 2000-4000 series, and the 4000 series wasn't THAT old). Nevermind Steam isn't exactly a large data intensive program to begin with...Older hardware with less RAM hence is a target market of Steam on Linux, and that includes all the folks who choose 32-bit for speed benefits (even if it's only an imaginary benefit).