It seems that all EBM machines use high temperature preheats to create parts that are stress free with no need for supports. On the other hand SLM machines using lasers seem to only preheat a fraction of the temperatures seen in EBM. SLM parts have stresses that need to be relieved with a post heat treatment, and in the build process also need support structures, whereas EMB doesn't have either of these. So why doesn't SLM operate at higher temperatures to achieve the same benefits? Is it a function of using a laser and the mechanics of operating it within these high temperature build envelopes? Is it something completely else?
from my knowledge, there's a lot of difference that can be part of an explanation, here are two of them :
First EBM work under very very low pressures (near to void) + electron are massive so when you hit the pouder you don't want it to get spread around. So you need to make it more compact. One way of achieving this is preheating (instead in SLM you're working under inert gaz atmosphere and with lasers so the need for preheating is not mendatory).
Second, EBM scan speed are much faster than laser scan speed (due to the mechanics of operating it as you said), wich allows you to preheat by scanning. Instead of laser technologies where you need to preheat with a heated plate. So you can have the last layers preheated instead of the first one, wich is much better as your build goes.
I hope this helped
Probably more of a side aspect, but still:
Related to the first part of Laurent Chougrani's answer, to my knowledge, under certain conditions there is a "dust" problem in EBM which can partly be solved by the scanning Laurent describes, as this is often controlled to create a weak bonding between the particles in the powder bed - weak meaning so weak that it does not make removal of excess powder too difficult, but still counteracts the "dusting" effect. Naturally, this, too, requires the heating brought in during scanning to be higher than in SLM.
There is also very instructive information explaining pros and cons of both methods in the following webinar: http://www.arcam.com/webinar-metron/