couple things you guys probably don't know but i have seen in a few topics.
1. is obviously speed can be increased by the devs through the engine, since frozenbyte owns their engine and servers.(which is a great thing since they dont lease them and have to ask permission to do things, and most importantly able to make everything they do fully compitable, one of reason an alpha that was out for 3 weeks was more stable than blizzards last three exp launch days.) but they got a cap i would assume for right now for programming reasons, balance tweeks they are working on, collisions(which is the number 1 destroyer of players ships, not pirates!), array of things, but they will increase the cap speed as game goes on.
2. you have to do your own research but there are ingame ways with your wrist com to take off the safety measures on your parts that will increase speed for slower ships and max out others, draw back is "wear and tear" and the fact you over push it, could cause your thrusters, generators, ect to explode. i myself have not tinkered with this but players do, and the vets have their own specific locks set for specific ships they know that can handle it.
i know this barely answers the question why is it set at 150, outside "devs need it like this for a reason right now" but hopefully some of the other info give you more insight on the speed setup. def check out the ability to adjust values on your parts to remove "safety locks".