Given the expense involved in licensing recognisable X-Men (and the need to strip over $10m from the budget less than two months before filming started) the writers were forced to scrape the bottom of the barrel as far as X-Men were concerned.
We went through such hell developing the script and which X-Men we
could keep and which we couldn’t and it just turned into a nightmare.
The studio would just say, ‘too expensive, too expensive, too
expensive’ to everyone. So finally we were like well, “What about
Negasonic Teenage Warhead” and they said, “Negasonic, what?”
Everything That Was Too Expensive to Include in Deadpool
Having winnowed the number of X-Men down to just two, they then needed to find a way to explain the bizarre lack of X-Men in a film explicitly set inside the X-Men Cinematic Universe.
Rather than contriving a plot-point to explain their absence, the writers went for the lazier option of simply having Wade 'hang a lampshade on it', telling audiences directly that it was a finance thing. Under normal circumstances this wouldn't work, but luckily Deadpool is renowned for breaking the fourth wall and speaking to viewers in a funny way.