why is the indicative used in this sentence rather than the subjunctive?

[spqr]

I translated this sentence from an old textbook of mine:

Praestat, inquiunt, si iam principatum Galliae obtinere non possumus, Gallorum quam Romanorum imperia perferre; I checked the translations of McDevitt and Bohn and they use the subjunctive form of possumus which I think makes more sense. This book I use is aimed at second year high school Latin students and frequently switches the grammar around and uses the indicative rather than the subjunctive as used in the original texts. Maybe the authors thought the students would understand the stories presented in the book easier but I find it more confusing actually.

[Constantinus Philo]

This is oratio recta

[seneca2008]

I am not sure what your question is? You translated an English sentence into Latin? If so it would be helpful to see it. The choice of indicative or subjunctive depends on what exactly you are trying to translate.

The sentence from which your text seems to be adapted is this:

"praestare, si iam principatum Galliae obtinere non possint, Gallorum quam Romanorum imperia praeferre" (Caes. Gal. 1.17.3)

Here possint is not the subjunctive form of "possumus".

Using the indicative changes the meaning. Its not a question of one being better than the other. It depends what you are trying to say.