There's an entry on the official site that attempts to answer the questions about the "missing" characters, and they list the characters which were considered, but didn't make it in the end :
Tatsunoko Side :
- Condor no Joe
- Yattaman #2
- Shurato
- Soul Taker
- Nakahara Komugi
- Daikyojin
- Beruku Katsue
- Tekkaman Evil
- Braiking Boss
Capcom Side :
- Leon
- Nash (Charlie)
- Date Masamune
- Arthur
- Ibuki
- Vega (M. Bison)
- ReiRei
- Zero Gouki
- Tiara Hime
- Nero
- June
- Rouge
- Tyrant
- Hinata
It's said that they decided of a 22-characters roster from the beginning, so it would seem like they had to narrow down the selection a bit. Finally, they give their main reasonings behind the omission of some characters :
1. Copyright issues
2. Licensed character which would be too difficult to translate into this game (concerns a large number of characters apparently)
3. Preferences
4. Overall balance
Oh, by the way, the producer also mentions how he would love to work on a sequel, and how he would like the game to sell a little more. I would think that both are related...
And here's the link :
http://www.capcom.co.jp/tatsucap/column11.html
[edit] Oh, yes, forgot about Tyrant and Hinata. Thanks Netto-kun.