The main answer here is simply lack of time. The server cost is negligilble in the scheme of things.
I installed and maintained gitlab voluntarily as part of all the rest of the Mer project infra almost single handed - and it wasnât easy. Gitlab had (has) a very high frequency update cycle and the amount of work per update was multiple hours. Then they would do security updates on top of that. Eventually I fell behind and Jolla also decided to move to github. At this point the service was badly outdated and we (well, mainly I) pushed to put our (limited) community manpower into the OBS revival.
As for the haste; there was a suggestion to leave the service running in RO mode but again I have been pushing to close it down ASAP - running a very outdated service with known security holes is a dis-service to the community IMHO and although we considered upgrading I advised that it would be better not to use any resource doing so - so you can blame me there.
I didnât look at how the redirects are being done but I admit that I assumed that the gitlab vs github url layout was not similar enough to support reliable rewritten urls especially when there will be issues and diff urls etc - Iâll suggest we spend a little time looking at that as I agree it would be useful in the short term (actually @ballock just mentioned this post in our meeting as Iâm typing and he is planning on doing some rewrite rules).
I hope this explains some of the reasoning and background.