From Dark to Light: the Universal Energy Quota and Portal Travel

There is something known as the Universal Energy Quota (UEQ), which is simply a measurement of the amount of life energy in existence. Senshi are manifestations of life energy, so when the Energy Quota is high, there are many senshi, and when it is low, there are fewer. Portals are warps in time-space that occur in any area with a high life energy level, and thus occur in any area with senshi.

In order to travel through a portal, a person must have certain amount of life energy, thus travel has traditionally been done by senshi or with a senshi present (senshi being higher-energy beings than normal people). Back in the days of the first great conflict, the UEQ was at an all-time high. There were tons of people and senshi and portals. Since the UEQ was high, traveling between portals was relatively easy, and anyone could do it while accompanied by a senshi.

Compared to the high point of the UEQ, the UEQ of modern-day Earth is very small. The portals are weak and unstable. This presents a problem for people wishing to travel through.

While in the past even the most minor senshi could use portals, the weakness of the current portals make it so only the most powerful senshi can pass through. Nowadays only Galaxia and Astra senshi are able to travel, as well as their DU equivalents (Stiria and Oor). The modern instability of the portals also adds another factor into the equation: the trip is one-way. Thus, the DU coming to Earth cannot get back (at least for the time being), and neither can the as-yet-unknown alien senshi.