Money, sir. I have a distinct lack of it.
I'm working as fast as I can on it, but cash is a huge issue.
For 1 meter of track, it costs me about $5. On the main line, I'm running about 120 feet of track, total, so that's approx. 40 meters (for the record, HO scale is 1/86, so IRL that would translate to 10,320 feet of track). That's $200 for the main line alone.
Switches run anywhere from $15 to $25 each. I have probably about 50 or so switches on the layout.
Cork for the roadbed beneath the track (which actually IS necessary, as cork keeps the track from expanding/contracting with the temperature, and all that expansion/contraction will fuck up the track if you aren't careful) runs around $15 for a 3x6 foot piece.
It's an expensive hobby, to say the least.