The show is not about ponies. It's about friendship. And I'll be honest, it doesn't look like that was on your mind while drawing this. He is the token male character, the second character introduced in the pilot, the lead character's closest, oldest, and most loyal friend, does more for her on a daily basis than anyone else could even if they tried, and being the only dragon in the cast makes him unique by default.
I just can't fathom any excuse for drawing this many ponies while forgetting to include one of the show's most important characters. Time and room certainly weren't factors. If this sounds like a rage rant, well, it kind of is. Pictures like this are infuriating, in spite of the time, talent, effort, and even feelings that went into them. It's pretty much impossible to enjoy the beautiful day, or to have any positive emotions for it, when the whole thing is basically a big slap in the face to Spike fans and, heck, anyone who merely possesses the knowledge that he's a main character.
It's nice to see this is one of your older submissions, but halfway through season two is still a little too late to justify not taking him seriously yet. Although I honestly wish I could enjoy it, I can't help but find it painful to look at. Sorry, but these are the thoughts and feelings this picture leaves me with.
Besides, you only made up a non-canon excuse that has nothing to do with why the artist chose not to draw him.