I definitely agree that it seems odd that they stopped so far short of 400 subjects, but there are so many inexplicably missing players (Lajoie, Wood, Alexander, Flick, McKechnie, Jackson, Carey, Waddell, Plank, Crawford, Cole, Coombs, just to name a few big ones) as well as examples from other sets where the T205 artwork was used for players NOT in the T205 set:
http://www.net54baseball.com/showthread.php?t=163810
It is quite plausible that there was another major series intended to be produced that simply never got off the ground for whatever reason. Given what we know from various contemporary writings, there was significant demand for the baseball player cards at that time. So I have to believe that the better option would have been to continue the baseball series with at least some of the aforementioned very popular players rather than change course and put 100 bird cards into the set instead. I mean, I like the Ruffed Grouse as much as the next guy, but I imagine I'd have much rather pulled a Nap Lajoie.