Long story short, and avoiding all the elitism in these sorts of threads that tell you to do normal mode (totally ignoring all the bullshit that goes along with pugging a new raid through group finder eg "LFM Norm ToS 920+ ilvl link AotC, and download our stupid obscure voice program because we're too stupid to used Discord, and pull 1.5m dps consistently.")
LFR is gated to keep you subbed, you'll wait and pay for those 2 months while you wait for the whole thing, and so will the other 5 million people waiting for LFR. It also appeases the vocal minority of hardcore raiders on the forums who make whinge threads everytime they see someone with an item with the same name as theirs but without the little "heroic" or "mythic" tag under it.
Edit: I feel it also should be pointed out that 99% of pugs of a new raid, will fail at or before the first boss. Unless they're being led by organised raiders who have read up on or tested the fights, it's just like running into LFR but the raid is tuned waaaay too high.