The Commonwealth was living far beyond it's means towards the end anyway; running something like a forty percent deficit. Military government was at least stable and orderly, but the rule of the major- generals was not sustainable in the long run, and they knew it.
I'm not convinced that power was ever really with the gentry at all; only the ability to obstruct and defy.
The monarchy was too firmly embedded in the framework of the law for order to be practical without massive rewriting of the statutes, and after Putney, real republican reform was all but impossible; what actually happened was just one damned expedient after another.
After the failure of parliament to come up with anything in 1648, which resulted in Pride's Purge (not coincidentally, some regiments including Pride's were owed up to thirteen months' back pay), the Rump lumbered on, decapitating the incorrigible Charles I in the process- another desperate expedient.
Charles had already tried to start a second round of civil wars (which some do dignify with the title) and proven that he would not stop intriguing to regain the full powers of his throne until forcibly stopped; parliament continually allowing him to do so was the main motive behind the purge. (That and back pay.)
The Second and Third Civil Wars, Ireland and the beginning of the Anglo- Dutch Wars get in the way, and all is expedient until the final dismissal of the Rump, which has produced little or nothing that could be called government- "you have sat here too long for the good you have been doing"-
and the attempt to produce a government of the godly, Barebones' Parliament- which also turns out to be a hopeless babbling shop and a complete failure, unable to cobble a republic together out of biblical quotations.
It is at this point that Cromwell becomes Lord Protector, after all other attempts at stability have failed, and it took considerable pressure to get him that far.
The Army would not have supported outright taking the crown, because it would have been a betrayal of everything they had fought for since 1642, and they would have stopped him- if he had needed to be stopped, because he felt the same way.
Cromwell was a religious radical but a social conservative, he would not have had the confidence of the army without being the puritan independent that he was, and being that he could hardly have taken the crown.