You can pay with PayPal...
Also, let's be honest here, you only want this game to be on Steam because you want to pick it up for $5 in a sale. (Nothing wrong with that per se, though.)
My problem with Matrix is similar - their lack of sales for games where I MIGHT be a potential customer, but not for the price they ask.
For games that are in my ballpark, I pay what they ask without grumbling over it - Distant Worlds and Panzer Corps are good examples.
But with games I'm less sure about, there's a pretty high "miss" ratio where I play a game for an hour or two and realize it's just not for me - their original pricing is often way too high to take this risk.
But hey, they slowly seem to come around - they're doing their weekly sales for older titles now, often with substantial discounts (even if it's on an relatively outrageous base price for years old games) and occasionally even do special sales - I picked up a number of titles at 50% off on eastern.
Also, Matrix doesn't seem to have access to a direct-link into Steam anyway - they tried to get Panzer Corps on, but it's been sitting in greenlight for forever now, while Valve happily accepts unfinished projects of games with 8bit graphics. Maybe one day...