Posted 10 June 2015 - 01:28 PM
Actually, Thel's forces are still very much using more classic designs; we've seen them in Halo: Escalation used by Thel's forces.
So it's likely a gameplay feature, make Jul's covenant look as they do, give Thel the more classic designs and you therefore have a fairly easy way to tell apart factions- imagine trying to distinguish friendly and enemy forces when they're all the same. Jul uses scavenged technology, whereas Thel has the factories on his side so it makes sense the two will look different.
Additionally, the UNSC underwent a major design change in Halo 4. Namely in the marines, foremostly. The ODST's and the Army (Reach)still retain their original styles very much. No point fixing something that isn't broken- nothing Jul uses is new, it's older tech than Thel uses.