Per Mashable, somebody can like you, want to be with you, and desire to reach out to you, but just isn't able to do so for whatever reason — you don't know what's going on in their life. Being told that "if he wanted to, he would" implies that he simply doesn't want to, which can be unnecessarily hurtful to hear when the timing is tactless, and has the chance of being completely wrong in any given situation. 

Socio Mix goes further, describing the phrase as "oversimplified and a little ableist" because sometimes people just aren't equipped with the skills or means they need to be able to do what they want — or what you want them to do, for the matter. Applying it to other aspects of your own life, there are other things you might very much want to do, but simply are not be able to. So, maybe he wants to text you, but just doesn't know how to — it's by no means a reflection on either of you. And remember, if things don't work out, it might be healthier to stay single, anyway.