I'm sure they tried many times to put psionics into sub-classes first.
They are doing it, but very reluctant.
It probably isn't your intent, but this comes across very strongly of confirmation bias. Evidence against the claim is suspect, or even somehow serves to support your claim. In the end, all we can say is that most of what they
have chosen to do thus far is subclasses, but they have explicitly committed to psionics having its own class for several months now. And,
if I recall correctly (which might not be the case), while they had originally been on a distinct kick of "condense EVERYTHING" in the early-mid playtest e.g. the "Mage" was going to absorb Sorcerer, Wizard, Warlock,
and Psion, that track was abandoned pretty quickly because it led to excessively uniform, indistinguishable characters--the first good evidence that they've planned, since well before release, to give psionics its own class.
Plus, they were perfectly willing to consider creating an entirely new, playtest Ranger class as part of its ongoing re-assessment. I really don't see them as "very reluctant" to create new classes. Sure, they probably
prefer making subclasses for a number of reasons (by leaving out the core class info they might drive sales of PHBs; subclasses have a much smaller design footprint and thus aren't as
difficult, etc.) But that doesn't mean that they're intentionally avoiding creating new classes as a sort of secret principle which psionics got a special and unique exception to.
In fact, I'd argue that the "subclasses are smaller and thus easier to design" explains the vast majority of it. It's the same thought process as "eh, I'm not going outside today and nobody's coming over, I'll just wear my pajamas" vs. "A friend/date/family member is coming over, I should look presentable." It's not that I
can't do those things, nor that I have any opposition to them (far from it, in fact). But when I know I'm going to be on my own for a whole day with no outside commitments, I see little point in dirtying a full set of regular clothes when I could just as easily wear pajamas. Should a need to properly dress arise--and it's both easy and natural for such events to occur, e.g. "crap I forgot to get milk, better go get some"--I'm perfectly happy to do so.
What if the WotC design team sees things in a similar way? Nothing to do with "reluctance" to create full classes, and merely favoring the easier subclass path whenever it would be sufficient to the task. For Psionics, they had already more or less determined it wasn't going to be. If we go a speculative step further, and assume that something similar to my "the Warlord was fully intended from the start, but fell through the cracks by accident" argument, then it would make a lot of sense that they haven't quite determined whether or not they'll give it a full class yet; even as late as August 2013, with only 3 months of public playtesting (and ~6-8 months of
any playtesting) left to go, the Warlord(-Fighter) still had martial healing, at least according to Mearls' tweets. If things changed enough in those last few months specifically for that subclass (as the Fighter overall took a terribly long time to hammer out), it's entirely possible that they are only now reaching the point of deciding, "Hmm, maybe it really does need its own class." The Psion took nearly as long, and that was without jury duty sapping their strength or the demands of regular publication distracting them from creating new designs.