The classic commentators ask the same question. Here are some of their answers:
Rabbi Chayim Atar tells us that in order to understand this, we must look carefully at their complaint:
"…when we sat by pots of meat, when we ate bread to our fill!"
The complaint was that they missed sitting by their pots and eating bread while the meat was cooking.
Killing a large animal affords a person an abundance of food at one time. After that, since they didn't port refrigerators around in the wilderness, it had to be salted and eaten cold. The Israelites wanted smaller animals so they could enjoy fresh meat on demand.
In addition, they missed eating bread, something which they had no access to in the barren desert.
In the complaint recorded in the Torah we can see both demands: A. they wanted smaller animals which would afford them fresh meals which they would eat right off the fire and B. they wanted bread to eat while the meat was cooking.
Alternatively, the Rabbi Avraham Ibn Ezra suggests that they had already eaten a large portion of their cattle. A this point, if they would all go to purchase more meat at once, the price would skyrocket beyond their means.
Please let me know if this helps,
Rabbi Menachem Posner