Why does the I.F. use old ships in the final battle at the Formic homeworld?

Question

In the final battle, the book says:

As for his own fleet, it consisted of twenty starships, each with only four fighters. He knew the four-fighter starships they were old-fashioned, sluggish, and the range of their Little Doctors was half that of the newer ones. Eighty fighters, against at least five thousand, perhaps ten thousand enemy ships.

Where are the other, newer ships, which were used in the previous battle? Why does the I.F. use old-fashioned, sluggish ships in such an important battle?

Answer 1

The home planet was the farthest one away (or one of them anyways). They took the longest to reach the planet.

Newer ships were sent to closer planets, so they had better technology, more fighters, etc.

"And the ships have been traveling for seventy years--"
"Some of them. and some for thirty years, and some for twenty. We make better ships now. We're learning how to play with space a little better... The first ships we sent to the most distance objectives, the more recent ships to the closer ones. Our timing was pretty good. They'll all be arriving in combat range within a few months of each other. Ender's Game - Ch. 13