The reason smoke currently takes two Ready actions to lay is because if it didn't your armoured vehicles could roll forward with a Move/Smoke each turn and make themselves - and any units hanging close to them or following them down a street - completely invulnerable to infantry and light vehicle attacks. Never mind that the Armoured vehicle dropping smoke can cheerfully shoot out without hassle. It might not be exactly realistic, but it's sort of necessary for game balance.
Still, if that's not to your liking, here are some suggestions for house rules (in no way official, since I just made them up just now):
1. Dropping smoke takes only a single Ready action, but provides Cover or Obscurement rather than blocking LOS.
2. Dropping smoke takes only a single Ready action and blocks LOS, but smoke launchers are much much more expensive. +100 points or something.
3. If you're willing to put up with added complexity, then you could say that each Ready action you take increases cover by one level - from Obscurement to Cover to blocked LOS. Where different vehicles use smoke all at once, where the areas overlap the cover level stacks.