Writing well is not just a matter of spending time and effort. Sitting down for 5 hours and expending great force of will is insufficient. One has to understand how to direct that effort; extensive knowledge about how to think and write well is required.
To create this knowledge involves a process of continuous self-improvement. One needs to care deeply about improving and always -- every day for years -- be on the lookout for ways to improve, and be implementing and refining strategies designed to learn and improve. 
Trying to learn to think and write well involves frequently making an effort to think and write well. Thoughtful writing is a byproduct.
On the other hand, writing without making a thoughtful effort is incompatible with always striving to learn and to write better. Writing shoddy posts indicates that one doesn't care strongly about bettering oneself in this area. 
 There are further requirements. For example, one must treat ideas and disagreements in such a way that if he is mistaken, he may find this out and change his mind. However, making an effort to learn and improve involves investigating what approaches are effective. Thus one can learn the further requirements early on in the process and doesn't have to know them in advance.
 People often make excuses. They are busy, they were just having a bad day, they could do better but they didn't give it their best effort, or emotions clouded their judgment. The fact is that everyone faces obstacles and encounters problems. Excuses are used by people with a victimhood mentality, who'd rather have something to blame than work on a solution. People whose top priority is to better themselves do not make excuses or denials; instead they try to overcome their problems and when they fail they try again and brainstorm ways to do better next time.