The most likely reason is that the armed forces in Ethiopia are not strong enough to achieve a successful military outcome, from their perspective, in the conflict in and around Tigray. This means they have to rely on the Eritrean military, which, as we know, has a long history of animosity toward the Tigrayans. Really, it's a combination of the two.
Quite honestly, my suspicion is also from some military analysis. I do not think the Ethiopian military has the capacity to stop the Eritreans coming into the country. It may well be that Abiy is calling on the Eritreans for assistance in his military campaign against Tigray, but at the same time, I do not think the Ethiopians are strong enough to hold the Eritreans away. Remember, they had a serious conflict in the past, and it was more or less a stalemate for many years.
It's a combination of those kinds of weaknesses of the Ethiopian military, as well as expediency on the part of the leadership of both countries against their common enemy, which is Tigray.