Because 4988 will not do anywhere near 2.5 amps (Motors rated max), or 2 amps for that matter without serious heatsink and fans. You could get the 4988 black edition boards from Pololu that do a little better and might get you all the way to 2 amps with heatsink and fans, or 1.2 without. They would be $10 each at pololu. The next step up would be the DRV8825 boards that will go up to 2.2 amps with heatsink and fan 1.5 amps without. Pololu sells them for $12 each. You could save a dollar each if you get the MD20a version but you'll have to hack the board a bit to get them to run on Sanguinololu.
Or you could get a beta board of Sanguish for the price of Pololu's alone and reach 2 amps easy. :)
Or you could get a beta board of Sanguish for the price of Pololu's alone and reach 2 amps easy. :)