The reason as to why theists call their God spaceless, timeless and immaterial is the recognition of the fact that if the universe has a beginning, then that beginning can never be from a source that already contained space, time, matter and energy.

Universe primarily means its space, time, matter and energy. Therefore when we say that the universe has a beginning, we mean to say that its space, time, matter and energy have a beginning. Now the question is: can the universe have a beginning from a source that already contained space, time, matter and energy? If the source already contained space, time, matter and energy, then that would mean that space, time, matter and energy were already there. If space, time, matter and energy were already there, then that would further mean that the universe was already there. If the universe was already there, then why do we again say that the universe has a beginning?

The above reasoning shows that if the universe has a beginning at all, then that beginning can never be from a source that already contained space, time, matter and energy because in that case it will imply that the universe was already there. Therefore the beginning of the universe will always mean that it can begin from zero space, zero time, zero matter and zero energy only. As the universe can begin from zero space, zero time, zero matter and zero energy only, so the total space, total time, total matter and total energy of the universe should always remain zero, as otherwise one will have to explain as to whence appear the extra space, extra time, extra matter and extra energy that were not already there at the beginning.

So for a universe having a beginning this question must have an answer: how does the total space-time of an ever-expanding universe always remain zero?

Only a beginningless, eternal universe will not give us any such trouble.