Because I don't like the Defiant. If I spec for an escort, then I am stuck with it for one full rank. At Tier 5, I have the choice between three. Also, Tier 3 has Two Curisers and two Science ships. Arguably, there is an additional escort, but it is in fact a varient of the standard escort (and an extra power due to the ensign slot being upgraded to Lt.)
First, choice of three? Fleet Escort, Advanced Escort (or identical looking MVAE) and the Defiant. Since you don't want the Defiant surely it's a choice of two.
I went through Tier 3 before they had a choice, so I'll give you that they shoved the Excelsior and Nebula into the Ship Trees there but that is an exception not the rule. Trust me I never wanted to fly the Cheyenne Class or any of those goofy looking four Nacelle Cruisers, but I wanted a cruiser so that is what I flew.
I don't see a demand for the Yeager, other than it's removal from many peoples memories. Your probably the only person on these boards who wants to actually fly a Yeager class.
Here's the thing from a logical design point of view, there is no reason to develop a new Tier 4 or lower anything. You get through Tier 4 and you play with Tier 5, how about you just fly the defiant, and get through the levels quick as possible so you can get the Fleet Escort or the AE. And in all seriousness the Defiant while not a traditional Starfleet vessel isn't a bad looking ship. So either make do with the Akira or try the Galaxy for ten levels.