Not really a rules question so this thread will probably get moved
Really it just comes down to the fact that Space Marines are a very popular army that have been around for decades. In that time that have accumulated loads of units that fill every possible niche (other than weak, disposable chaff I guess). The lore and the chapter system gives them loads of named characters, each with little tweaks to make them interesting.
The Adeptus Mechanicus on the other hand are pretty new, at least in terms of being able to field an army full of models. They therefore have fewer options to pick from. If
GW had introduced them as a newly-playable army with ten different configurations of leader models then people would have been overwhelmed. However, they have got a few new units recently (proper transports) and are getting several more with one of the upcoming Psychic Awakening books (cavalry, flyers and 'jump pack' dudes).