Because it separates players into groups based on disposable income. The cards allow you to do more "cool things" based on the amount of real world money you spend on them. As a person with a small gaming budget, and who abandoned CCG in 2000, I have no interest or money to invest in a deck. So I resent the fact that a person could build the same character I have, but with the equivalent of an extra utility power which could be used multiple times in an encounter.
When running a game, I don't want to discourage my more casual players from playing the game, or slow down combat further with the additional set of minor tactical options. Some of my players would handle it quickly, others not so much.
While I can't speak for other players, the cards do not slow me down at all, requiring only a moment of additional thought based on the card in my hand if freshly drawn, otherwise, i've had the entire time during other player's turn to decide how I want to use the card.
Without venturing too deeply into some form of political or sociological tangent, is it one's job as DM to limit content allowed in a game based on the financial capabilites of one's self or the other players? Does a DM have reasonable justification to exclude 1st party material such as PHB 2, 3, the Powers books, adventurers vaults, or the essentials lines based soley on other players have not yet had the oppurtunity to buy it themselves and only have PHB 1? Is there reasonable justification for first party DDI material to be excluded since it's available only to those that wish to pay for DDI. There, one has clear cut feats, classes, races, powers and actual game changing character creation options available only to those that pay a premium.
If one is not willing to exclude DDI character creation content, or supplements beyond PHB1, it is hypocrisy to then exclude something like Fortune Cards that grant minor buffs simply because it costs money, or those with larger expendable income can afford to purchase cards of a rarer nature by bypassing randomized booster packs.
That goes for any game system that has more than just a core rulebook. If one wants extra options, such as those found in Pathfinder's Advanced Player's Guide, that person pays to get those options and advantages beyond the core options that everyone else has. Other players have that same ability if they so choose to invest their money as such. If they choose not to exercise it, one does not penalize or ostracize the person that did choose to buy those extra options.
If a source of 1st party options does not interfere with the campaign world (for instance, reasonable arguments can be made about excluding psionic or another power source as it does not mesh with a given campaign world, or specific deities, specific themes or specific artifacts) then I can see no reasonable justification to deny payed content in any cooperative system.